The conditioning-testing P50 paradigm is used to demonstrate sensory gating of responsiveness to auditory stimuli. Sensory gating is measured in terms of a suppression of the second (test) P50 component in that paradigm. The time course of sensory gating can be determined by examining subjects' gating of responsiveness to stimuli repeated at various interpair intervals. In the present study auditory evoked potentials were recorded using a paired click, conditioning-testing P50 paradigm in 11 normal subjects with no family history of any psychotic disorder. Recordings were made at conditioning-testing intervals of 250 ms, 500 ms, 750 ms and 1000 ms. Whereas the grand averages of the P50 conditioning-testing response for the 250- and 500-ms intervals were 3.07% and 37.2%, respectively, indicating almost complete suppression, the grand averages of the ratios for the 750- and 1000-ms intervals were 114.35% and 92.92%, respectively, indicating little or no suppression. There was significant correlation in the C-T ratios with the increasing intervals. Our results suggest that the mechanism(s) responsible for sensory gating is activated mostly during the 500 ms after stimulus presentation. Other gating mechanism(s) functioning at longer intervals appear to be uncertain. (C) 2001 Elsevier Science B.V. All rights reserved.