Most normal subjects suppress the P50 evoked response to repeated auditory stimulus when paired stimuli are presented, 500 ms apart. The ratio of the amplitude of the response to the second stimulus to that of the first forms a quantitative measure of the activity of a central inhibitory sensory gating mechanism. The variance of this measure in a group of monozygotic and dizygotic twin pairs was determined. The calculation of heritability (h2) indicates that a significant portion of the variance (0.44 or greater) is due to heritable factors. This study supports the use of the inhibitory gating of the P50 response as a phenotype for genetic studies of schizophrenia in which a P50 gating abnormality has been identified.