1. The iontophoretic application of the alkaloid bicuculline to neurones in area 17 of the cat's visual cortex effectively antagonized the inhibitory action of iontophoretically applied GABA in fifty-four out of sixty-two neurones examined. It had little or no effect on the inhibitory action of iontophoretically applied glycine. 2. At the stage that the iontophoretic application of bicuculline blocked the inhibitory action of GABA it also reduced or blocked visually evoked inhibitory influences acting on forty-three of the fifty-four cells. This effect on visually evoked inhibition was not reproduced by simply raising the neural spontaneous activity with iontophoretically applied glutamate. 3. For those seven neurones where the iontophoresis of bicuculline failed to block the inhibitory action of iontophoretically applied GABA it also failed to produce any change in visually evoked inhibition. 4. In all cases where a visually evoked inhibition of a cells resting discharge was reduced by the iontophoretic application of bicuculline, the inhibitory response was replaced by an excitatory response. The application of bicuculline also revealed excitatory responses to certain of the visual stimuli that previously appeared to exert neither inhibitory nor excitatory effects on a cell, and often where cells normally exhibited small excitatory responses it produced large increases in the magnitude of the evoked response. 5. These results indicate that the normal responses of the neurones examined in the present work, to the particular visual stimuli used, reflect an interaction between simultaneously evoked excitatory and inhibitory inputs. It is suggested that the iontophoretic application of bicuculline by blocking or reducing the inhibitory input moves the balance between the inputs in favour of the excitatory input. 6. The present results support the view that GABA is an inhibitory transmitter in the visual cortex.