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OBJECTIVES: Deficient prepulse inhibition (PPI) of startle in schizophrenia patients and unaffected family members may be a useful endophenotype in studies seeking to identify vulnerability genes for schizophrenia. Before expanding such studies to include Pacific Rim populations with distinct genetic origins compared with North American Caucasian populations, we examined PPI and related startle measures in normal North American Caucasian and Asian men. METHODS: One hundred and seventy-four consecutive carefully screened right-handed male 1835 year olds completed tests of startle and PPI using bilateral electromyography measures of orbicularis oculi. Subjects identified their racial background. RESULTS: Startle in Caucasian subjects was of significantly larger magnitude and shorter latency compared with Asian subjects; the percent PPI was significantly greater in Asian versus Caucasian subjects at 60 and 120?ms prepulse intervals. Group differences in PPI but not reflex latency were eliminated when groups were matched for startle magnitude on pulse alone trials. Caucasian-American and Asian-American groups did not differ significantly in resting blink rate, resting electromyography activity, or startle reflex habituation. CONCLUSIONS: Robust phenotypic differences in acoustic startle magnitude and latency between Caucasian-American and Asian-American populations must be considered in studies using startle and PPI as endophenotypes across these populations. [ABSTRACT FROM AUTHOR]