We studied the development of gamma-aminobutyric (GABA) mediated inhibitory processes of the lateral geniculate nucleus cells in kittens of various age groups, by measuring the effect of ionophoretic application of GABA and bicuculline on cell response to sinusoidal gratings. In young kittens (less than 30 days) we found very few Y cells only X cells and weakly responsive cells which fitted neither X nor Y classifications ('immature' cells). As with adult cells, GABA inhibits the visual response of young kitten cells. Simultaneous application of bicuculline restored responsiveness. The mean GABA current to silence cell response in young kittens was significantly higher than that obtained in adult cats. Application of bicuculline alone had little effect on young kitten X-cell responsiveness, either on the average discharge or on the amplitude of modulation to stimulation by sinusoidal gratings. For older kittens (40-45 days), bicuculline increased X-cell responsiveness, and the increase in responsiveness was dependent on stimulus contrast and spatial frequency. However, the increased responsiveness was less than that for adults. At 100 days the changes of slope of X-cell contrast-response curves during bicuculline application were similar to those observed for adult X cells. We conclude that, although GABA receptors may be present at 30 days, the GABA mediated inhibitory system does not begin to function until about 45 days and does not mature fully until about 100 days.