When tested with pairs of brief visual stimuli, neurons of the primary visual cortex of the cat show a long-lasting, orientation-selective suppression, termed “paired-pulse suppression.” The hypothesis that this suppression is due to GABAA-mediated inhibition was tested by performing temporal interaction tests before, during, and after iontophoretic application of the selective antagonist bicuculline methiodide (BMI). In keeping with previous reports, BMI elevated the spontaneous and evoked firing rates of cortical neurons, and altered basic receptive field properties. Under the influence of BMI, most neurons showed a reduced or abolished selectivity for stimulus orientation and direction of movement. The effects on orientation selectivity required higher ejection currents than did the effects on directional selectivity. At high ejection currents, most cells did lose selectivity for the orientation of a moving stimulus, but retained some selectivity for the orientation of a stationary stimulus. BMI, even at very high ejection currents, did not abolish paired-pulse suppression. In some cells, BMI enhanced or prolonged paired-pulse suppression. In further experiments, temporal interaction tests were performed in which one or the other of the component stimuli was replaced with a pharmacological stimulus (a pulse of glutamate or potassium). A pharmacological stimulus did not produce suppression of the response to a subsequent visual stimulus, nor did a visual stimulus suppress the response to a subsequent pharmacological stimulus. Paired-pulse suppression occurred only when both stimuli were visual. Taken together with previous results, the present data indicate that paired-pulse suppression is most likely due to a presynaptic mechanism.