Excitatory postsynaptic potentials (EPSPs) were evoked in principal cells of the cat's dorsal lateral geniculate nucleus by electrical stimulation of cortico-geniculate fibres. The EPSPs had a pronounced frequency sensitivity. They were barely detectable at stimulation frequencies below 3 Hz but increased dramatically in size at higher frequencies. At 30-50 Hz their amplitude typically exceeded that of EPSPs from optic tract fibres. A prominent EPSP potentiation was also obtained with pair pulse stimulation. The findings are discussed in relation to the hypothesis that the cortico-geniculate system serves as a variable gain regulator for the visual input to the cortex.