The effects of caffeine on mood and memory tasks were investigated in a double-blind study of 95 healthy young adults who were randomly assigned to three doses: 0, 200, and 400 mg of oral administration of caffeine. Subjects completed a battery of tasks once predrug and several times postdrug. Caffeine, in general, showed nonsignificant effects on cognitive, learning, and memory performance. The exception is that 200 mg caffeine facilitated performance on the relatively more difficult cancellation (addition and multiplication) tasks than the digit cancellation task. In addition caffeine decreased boredom and relaxation, and increased other ratings of subjective moods--anxiousness, tenseness, and nervousness. The reduction in boredom or fatigue is associated with the repetitive nature of the task and the period of time during which the tasks were repeated. In general, high-to-moderate users of caffeine recalled more words than low users, particularly at the beginning of the lists. Results are discussed in terms of the effects of caffeine on task difficulty and sensitivity and the relative potency of caffeine doses on behavior. User effect is suggested as an important consideration of memory assessment.