Theobromine versus placebo discrimination and caffeine versus placebo discrimination were studied in two consecutive experiments in seven volunteers who abstained from methylxanthines. Daily sessions involved PO double-blind ingestion of two sets of capsules sequentially, one of which contained drug and the other placebo. Subjects attempted to identify, and were later informed, which set of capsules contained the drug. In each experiment subjects were exposed to progressively lower doses. Five subjects acquired the theobromine discrimination; the lowest dose discriminated ranged from 100 to 560 mg. All seven subjects acquired the caffeine discrimination; the lowest dose discriminated ranged from 1.8 to 178 mg. A final experiment evaluated subjective effect ratings following 560 mg theobromine, 178 mg caffeine and placebo, which were administered double-blind in capsules once daily, five times each in mixed sequence. Caffeine produced changes in both group and individual ratings (e.g. increased well-being, energy, social disposition and alert). Theobromine did not produce changes in group ratings but changed ratings in some subjects. Across subjects, sensitivity to caffeine discriminative effects in the discrimination experiment correlated significantly with the number and magnitude of caffeine subjective effects in the final experiment. This study documents modest discriminative effects of theobromine in humans, but the basis of the discrimination is unclear. This study suggests that commonly consumed cocoa products contain behaviorally active doses of caffeine and possibly theobromine.