diff --git a/lib/mongo/collection.rb b/lib/mongo/collection.rb
index 7fde81c..00f54b4 100644
--- a/lib/mongo/collection.rb
+++ b/lib/mongo/collection.rb
@@ -130,7 +130,7 @@ module Mongo
       end
       raise RuntimeError, "Unknown options [#{options.inspect}]" unless options.empty?
 
-      cursor = @db.query(self, Query.new(selector, fields, skip, limit, sort, hint, snapshot, timeout))
+      cursor = @db.query(self, Query.new(selector, fields, skip, limit, sort, hint, snapshot, timeout, @db.slave_ok?))
       if block_given?
         yield cursor
         cursor.close()
diff --git a/lib/mongo/db.rb b/lib/mongo/db.rb
index 22b49c5..feeec2b 100644
--- a/lib/mongo/db.rb
+++ b/lib/mongo/db.rb
@@ -160,6 +160,9 @@ module Mongo
           is_master = master?
           @semaphore.lock if semaphore_is_locked
 
+          if !@slave_ok && !is_master
+            raise ConfigurationError, "Trying to connect directly to slave; if this is what you want, specify :slave_ok => true."
+          end
           @slave_ok || is_master
         rescue SocketError, SystemCallError, IOError => ex
           close if @socket
diff --git a/lib/mongo/errors.rb b/lib/mongo/errors.rb
index 3eefbe6..7fd6b43 100644
--- a/lib/mongo/errors.rb
+++ b/lib/mongo/errors.rb
@@ -15,6 +15,12 @@
 # Exceptions raised by the MongoDB driver.
 
 module Mongo
+  # Generic Mongo Ruby Driver exception class.
+  class MongoRubyError < StandardError; end
+
+  # Raised when configuration options cause connections, queries, etc., to fail.
+  class ConfigurationError < MongoRubyError; end
+
   # Raised when a database operation fails.
   class OperationFailure < RuntimeError; end
 
diff --git a/lib/mongo/message/opcodes.rb b/lib/mongo/message/opcodes.rb
index 235ecd7..4a19bc9 100644
--- a/lib/mongo/message/opcodes.rb
+++ b/lib/mongo/message/opcodes.rb
@@ -25,5 +25,6 @@ module Mongo
   OP_DELETE = 2006
   OP_KILL_CURSORS = 2007
 
+  OP_QUERY_SLAVE_OK = 4
   OP_QUERY_NO_CURSOR_TIMEOUT = 16
 end
diff --git a/lib/mongo/query.rb b/lib/mongo/query.rb
index e8392d1..af056c3 100644
--- a/lib/mongo/query.rb
+++ b/lib/mongo/query.rb
@@ -67,9 +67,9 @@ module Mongo
     #             the normal cursor timeout behavior of the mongod process. 
     #             When +false+, the returned cursor will never timeout. Care should 
     #             be taken to ensure that cursors with timeout disabled are properly closed.
-    def initialize(sel={}, return_fields=nil, number_to_skip=0, number_to_return=0, order_by=nil, hint=nil, snapshot=nil, timeout=true)
-      @number_to_skip, @number_to_return, @order_by, @hint, @snapshot, @timeout =
-        number_to_skip, number_to_return, order_by, hint, snapshot, timeout
+    def initialize(sel={}, return_fields=nil, number_to_skip=0, number_to_return=0, order_by=nil, hint=nil, snapshot=nil, timeout=true, slave_ok=false)
+      @number_to_skip, @number_to_return, @order_by, @hint, @snapshot, @timeout, @slave_ok =
+        number_to_skip, number_to_return, order_by, hint, snapshot, timeout, slave_ok
       @explain = nil
       self.selector = sel
       self.fields = return_fields
@@ -121,7 +121,9 @@ module Mongo
     # Returns an integer indicating which query options have been selected.
     # See http://www.mongodb.org/display/DOCS/Mongo+Wire+Protocol#MongoWireProtocol-OPQUERY
     def query_opts
-      @timeout ? 0 : OP_QUERY_NO_CURSOR_TIMEOUT
+      timeout  = @timeout ? 0 : OP_QUERY_NO_CURSOR_TIMEOUT
+      slave_ok = @slave_ok ? OP_QUERY_SLAVE_OK : 0 
+      slave_ok + timeout
     end
 
     def to_s
diff --git a/test/messages/test_query_message.rb b/test/messages/test_query_message.rb
index ea481ef..572a1f9 100644
--- a/test/messages/test_query_message.rb
+++ b/test/messages/test_query_message.rb
@@ -37,4 +37,22 @@ class TestQueryMessage < Test::Unit::TestCase
     assert_equal 16, buf[16]
   end
 
+  def test_timeout_opcodes
+    @timeout       = true
+    @slave_ok      = true
+    @query         = Query.new({}, nil, 0, 0, nil, nil, nil, @timeout, @slave_ok)
+    @query_message = QueryMessage.new('db', 'collection', @query)
+    buf = @query_message.buf.instance_variable_get(:@buf)
+    assert_equal 4, buf[16]
+
+
+    @timeout       = false
+    @slave_ok      = true
+    @query         = Query.new({}, nil, 0, 0, nil, nil, nil, @timeout, @slave_ok)
+    @query_message = QueryMessage.new('db', 'collection', @query)
+    buf = @query_message.buf.instance_variable_get(:@buf)
+    assert_equal 20, buf[16]
+  end
+
+
 end
diff --git a/test/test_query.rb b/test/test_query.rb
index 119c411..0210dfa 100644
--- a/test/test_query.rb
+++ b/test/test_query.rb
@@ -33,4 +33,22 @@ class TestQuery < Test::Unit::TestCase
     assert_equal 16, @query.query_opts
   end
 
+  def test_slave_ok_opcodes
+    @slave_ok = true
+    @query   = Query.new({}, nil, 0, 0, nil, nil, nil, true, @slave_ok)
+    assert_equal 4, @query.query_opts
+   
+
+    @slave_ok = false
+    @query   = Query.new({}, nil, 0, 0, nil, nil, nil, true, @slave_ok)
+    assert_equal 0, @query.query_opts
+  end
+
+  def test_combined_opcodes
+    @timeout  = false
+    @slave_ok = true
+    @query   = Query.new({}, nil, 0, 0, nil, nil, nil, @timeout, @slave_ok)
+    assert_equal 20, @query.query_opts
+  end
+
 end
diff --git a/test/test_slave_connection.rb b/test/test_slave_connection.rb
new file mode 100644
index 0000000..1863c87
--- /dev/null
+++ b/test/test_slave_connection.rb
@@ -0,0 +1,42 @@
+$LOAD_PATH[0,0] = File.join(File.dirname(__FILE__), '..', 'lib')
+require 'mongo'
+require 'test/unit'
+
+# NOTE: these tests are run only if we can connect to a single MongoDB in slave mode.
+class SlaveConnectionTest < Test::Unit::TestCase
+  include Mongo
+
+  def self.connect_to_slave
+    @@host = ENV['MONGO_RUBY_DRIVER_HOST'] || 'localhost'
+    @@port = ENV['MONGO_RUBY_DRIVER_PORT'] || Connection::DEFAULT_PORT
+    db = Connection.new(@@host, @@port, :slave_ok => true).db('ruby-mongo-demo')
+    !db.master?
+  end
+
+  if self.connect_to_slave
+    puts "Connected to slave; running slave tests."
+
+    def test_connect_to_slave
+      assert_raise Mongo::ConfigurationError do 
+        @db = Connection.new(@@host, @@port, :slave_ok => false).db('ruby-mongo-demo')
+      end
+    end
+
+    def test_slave_ok_sent_to_queries
+      @db   = Connection.new(@@host, @@port, :slave_ok => true).db('ruby-mongo-demo')
+      @coll = @db['test-collection']
+      @cursor = @coll.find({})
+      assert_equal true, @cursor.query.instance_variable_get(:@slave_ok)
+    end
+  else
+    puts "Not connected to slave; skipping slave connection tests."
+
+    def test_slave_ok_false_on_queries
+      @db   = Connection.new(@@host, @@port).db('ruby-mongo-demo')
+      @coll = @db['test-collection']
+      @cursor = @coll.find({})
+      assert_nil @cursor.query.instance_variable_get(:@slave_ok)
+    end
+  end
+
+end