A mutually exclusive choice procedure was used to evaluate the reinforcing effects of caffeine in eleven normal subjects with histories of regular caffeine consumption (mean 256mg/day). Subjects participated for 24 weeks; each week consisted of three consecutive daily sessions (two sampling days followed by a choice day) during which subjects were required to abstain from dietary sources of caffeine. On each sampling day subjects ingested four capsules, one every 2h. Capsules contained placebo on one sampling day and caffeine (50 or 100mg/capsule) on the other sampling day. Placebo and caffeine were associated with different color-coded capsules. At the beginning of the choice day subjects chose one of the two color-coded capsules they wished to take on that day; they were required to ingest one capsule and, thereafter, they could ingest up to six additional capsules of the same color throughout the day. Across subjects and dose, caffeine was chosen over placebo on 80% of choice occasions; nine of 11 subjects (82%) chose caffeine on more than 70% of choice occasions and caffeine choice was replicable despite changes in capsule colors across blocks. This is the first study in a relatively uncontrolled outpatient context to demonstrate significant caffeine reinforcement in the majority of normal subjects.