Prepulse inhibition (PPI) is a measure of the influence of a stimulus (S1) on the response elicited by a second stimulus (S2) occurring shortly afterwards. Most S1/S2 measures of gating have used behavioral startle and the P50 event-related potential (ERP) amplitudes to detect PPI in a simple paired stimulus paradigm. Aims: Here we report on two behavioral (EOG and reaction time, RT) and 5 ERP measures of PPI where S2 was the target or standard in an oddball discrimination. Subjects were 21 healthy controls (CON), 11 obsessive-compulsive (OCD) and 9 schizophrenic patients (SCH). Results: The prepulse 100ms before S2 induced fewer omission errors and longer RTs compared to a 500ms S1-S2 interval in all subjects. PPI was also evident in EOG, P50, N1, P3 but not P2 or N2 amplitudes of CON subjects. SCH patients showed attenuation of PPI on the same measures. OCD patients were characterized only by their slow RT and a marginal attenuation of EOG-PPI. Discussion: A correlational analysis implied separate relationships of ERP indices of PPI to the cognitive and psychomotor consequences of the prepulse on behavioral and discrimination responses. However SCH patients showed a general rather than a specific impairment of these indices.