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An experiment was conducted to investigate the effect of caffeine on the vigilance performance of introvert and extravert non-coffee drinkers. Forty-seven introverts and 43 extraverts participated in this study; all were female university students who do not drink coffee. Participants in both groups were given caffeinated regular coffee with approximately 140 mg of caffeine, and a coffee-like caffeine-free beverage in two separate sessions and in random order. Forty-five to 60 minutes later participants were tested using a computerized vigilance task and their performance was measured in d prime(d´). There was a significant potentiating effect of caffeine on vigilance in both introverts and extraverts. There was no significant effect of personality type or interaction between caffeine and personality type. Alterations in the parameters of the experiment are suggested for future research.