Gating of visual and auditory evoked responses was assessed in chronic schizophrenic patients treated with neuroleptic drugs. Middle latency components of the visual evoked response (N90-P130) were recorded at the occiput after flash stimulus. Possible inhibitory mechanisms of sensory gating were assessed in a conditioning-testing paradigm by measuring the change in amplitude of response to a second stimulus, relative to the response to the first stimulus. Simultaneous electrooculograms were recorded to detect contamination of recordings by eye movement. Neither schizophrenic patients nor normal control subjects demonstrated significant suppression of visual evoked responses in the conditioning-testing paradigm. These results differed markedly from similar measurements of a middle latency component of the auditory evoked response (P50) recorded using the same conditioning-testing paradigm in these subjects. Normal controls showed significant decrements of the P50 response to the second auditory stimulus (mean decrement over 80%), whereas schizophrenic patients failed to show a significant decrement (mean less than 40%). This finding for auditory evoked responses replicated previous studies of normal and schizophrenic subjects. Multiple conditioning stimuli were substituted for the single conditioning stimulus used previously in an attempt to enhance gating of auditory responses, but suppression of the P50 test response did not increase in either normal or schizophrenics.