Modulation of cortical firing rate is a major factor in defining cortical filter properties. Active response suppression (inhibition) is seen whenever cortical cells are exposed to grating stimuli that are non-optimal, in either the domain of orientation or spatial frequency. Responses are also reduced by pre-exposure to gratings of high contrast. The first phenomenon is termed spatially dependent inhibition, the second contrast gain control. We have explored the physiological basis for these two phenomena in striate cortical cells of the anesthetized cat. Sequences of spikes in responses show bursts characterized by interspike intervals of 8 msec or less. Both burst frequency and burst length depend on average firing rate, but at a given firing rate burst length is lower for non-optimal orientations. Burst length is also shortened by local injection of GABA. Burst length modulation is not seen in the case of contrast gain control. These results support the existence of two independent mechanisms for modulating cortical responsiveness. A GABA-ergic mechanism that shortens spike bursts is invoked by presentation of spatially non-optimal stimuli. Response normalization after presentation of high contrasts does not affect burst length and is not affected by GABA.