The acoustic startle reflex is normally inhibited when the startling stimulus is preceded by a weak prepulse. We studied "prepulse inhibition" (PPI) to assess potential gender differences in this operational measure of sensorimotor gating. A review of data from our previously published studies in psychiatric patients and normal controls indicated that startle in women was less inhibited by weak prepulses than was startle in men, and that this gender difference narrowed when stronger prepulses were used to elicit maximal levels of PPI. Based on these observations, new subjects were selected for study using our established criteria to exclude individuals with psychiatric disorders, substance abuse, or serious neurologic or medical illness. Replicating our preliminary observation, women exhibited significantly less PPI than men, with the gender difference being most notable under conditions with weak prepulses. Potentially confounding variables, including electrode impedance, startle amplitude, habituation and latency did not differ between men and women. PPI was then measured in male and female rats, where no gender differences were noted. These findings identify significant gender differences in PPI in humans, and suggest that inhibitory "gating" processes are more robust in men than in women, when assessed using a sensitive range of inhibitory stimuli.