SIMPLE cells within layer IV of the cat primary visual cortex are selective for lines of a specific orientation, It has been proposed that their receptive-field properties are established by the pattern of connections that they receive from the lateral geniculate nucleus (LGN) of the thalamus(1-5). Thalamic inputs, however, represent only a small proportion of the synapses made onto simple cells(6-8), and others have argued that corticocortical connections are likely to be important in shaping simple-cell response properties(9,11). Here we describe a mechanism that might be involved in selectively strengthening the effect of thalamic inputs, We show that neighbouring geniculate neurons with overlapping receptive fields of the same type (on-centre or off-centre) often fire spikes that are synchronized to within 1 millisecond, Moreover, these neurons often project to a common cortical target neuron where synchronous spikes are more effective in evoking a postsynaptic response. We propose that precisely correlated firing within a group of geniculate neurons could serve to reinforce the thalamic input to cortical simple cells.