Eight healthy male volunteers participated in four experimental sessions in which they ingested one of the following drugs: ranitidine hydrochloride (150 mg), cimetidine hydrochloride (400 mg), thioridazine hydrochloride (50 mg), placebo (lactose). Drugs were allocated to subjects and sessions in a double-blind fashion, according to a balanced cross-over design. The subjects' mood state and psychomotor performance were assessed 1 and 3 h after drug taking. Mood state was measured using a battery of visual analogue scales, and psychomotor performance using pencil-and-paper tests, critical flicker fusion frequency, wire-maze tracing and tapping. Ranitidine and cimetidine had no significant effect on subjectively rated alertness, whereas thioridazine caused a significant decrease in alertness. Ranitidine and cimetidine had no significant effect on performance on the pencil-and-paper tests (digit cancellation, digit symbol substitution, symbol copying), whereas thioridazine caused a significant decrement on these tests. Ranitidine and cimetidine had no significant effect on critical flicker fusion frequency, wire-maze tracing, and tapping rate. Thioridazine caused a significant impairment of psychomotor performance as evidenced by all the instrumental tests. It is concluded that, in contrast to thioridazine and similarly to cimetidine, ranitidine has little effect on subjectively rated alertness and psychomotor performance in healthy volunteers.