Research in this laboratory showed that caffeine (4 mg/kg) delays habituation of the acoustic startle reflex in humans. The present study examined the effects of 2- and 6-mg/kg doses of caffeine on acoustic startle habituation in moderate-high and low caffeine users. Eyeblink responses to 30 trials of 85-dB noise stimuli were measured beginning 30 min after oral ingestion of either placebo or 2 or 6 mg/kg of caffeine. The 2-mg/kg dose of caffeine delayed startle habituation in both moderate-high and low caffeine users. The 6-mg/kg dose produced no differential effects on startle responding from placebo. In moderate-high users, following habituation, startle responding was smaller in the placebo condition compared to both caffeine conditions. In low users there were no differences in posthabituation responding between doses, suggesting that this dose effect is dependent on a history of chronic caffeine usage.