Abstract: A test apparatus (12) and method (110) are provided for inserting predetermined packet loss into a data flow between a plurality of hosts on a packet-switched network. The test apparatus (12) can include a first network interface (20), a second network interface (22) and a packet filter (24). The first network interface (20) receives a sequence of data packets from a source host (14) under test. Incoming data packets are passed to the packet filter (24), which selectively discards predetermined ones of the data packets to generate a reduced sequence of data packets. The second network interface (22) transfers the reduced sequence of data packets over the network (18) to the destination host (16). The test apparatus (12) can be used accurately to measure the effect of packet loss on media applications transmitting real-time data, such as voice and audio, over packet-switched networks.