We performed experiments in the cat geniculocortical pathway, in vivo, to examine how presynaptic spikes interact to influence the firing of postsynaptic targets. In particular, we asked (1) how do multiple spikes from a single presynaptic neuron interact to influence the firing of a postsynaptic target (homosynaptic interactions), (2) how do spikes from two different presynaptic neurons interact (heterosynaptic interactions), and (3) what is the time course of homosynaptic and heterosynaptic interactions? We found that both homosynaptic and heterosynaptic interactions increase the likelihood of driving a postsynaptic spike, although with different time courses. For two spikes traveling down a single geniculate axon, the second spike is more effective than the first for approximately 15 msec. For two spikes on separate axons, the interaction is faster ( approximately 7 msec duration, approximately 2.5 msec time constant). Thus changes in firing rate are perhaps best relayed by homosynaptic interactions, whereas heterosynaptic interactions may help detect coincident spikes from different thalamic inputs.