Introduction: A measure of auditory prepulse inhibition (PPI) or sensory gating is the reduction of the scalp-recorded P1 event-related potential (ERP) after a sound that is preceded by 100-300 ms by a click as prepulse. This measure of sensory gating was adapted to study the effect of a prepulse on processing tones that were part of a "Go/no-go" discrimination. Methods: ERPs were recorded at right and left frontal and temporal sites on the scalp of groups of patients with schizophrenia (SCH, 8), obsessive compulsive disorder (OCD, 10) and healthy controls (CON, 19). Results: a) Both patient groups responded slower and showed more errors of omission than controls. b) A prepulse presented 100 ms (but not at 500 ms) before either tone reduced the P1 or P50 ERP amplitude on healthy controls, but showed a right temporal shift in the SCH patients. c) If the tone was the 'no-go' stimulus in the tone discrimination the prepulse reduced the N1 amplitude in both normal controls and SCH patients. This N100 amplitude was similar in records from over the left and right hemisphere of controls but was shifted to right temporal sites in the SCH group. d) OCD patients showed a relative predominance of the left hemisphere in the gating of frontal P1, N100 and P300 components, but early gating was not significantly impaired in this group. Conclusions: These results show a reduction of a PPI-like effect on early processing (e.g. P50/P1) that is more marked in the left hemisphere of SCH patients, and may affect channel selection for processing information about task-relevant sounds (e.g. N100)