Five female subjects possessing equal normal (established normal) reaction time, equal age, height and weight were used in this investigation. Each subject was habituated to one cup of coffee per day prior to the experimental period. By means of Metfessel's psychodometer the reaction time of each subject was studied under normal, caffeine-treated, coffee-treated, and capsulated starch control conditions. Results show that both caffeine (3.3 to 3.6 mgms/kgm) and the equivalent quantity in black coffee reduce the reaction time to the maximum extent of 8% and 4% respectively. Some reduction in reaction time is maintained for at least a 3-hour period. The caffeine alkaloid was more effective than the coffee beverage except during the first half hour, when the reverse order was observed. The most effective period was 1½ to 2 hours after treatment. (PsycINFO Database Record (c) 2006 APA, all rights reserved)