The influence of thioridazine (1.0 and 1.5 mg/kg body wt.) on human performance was studied in healthy subjects in a double-blind trial vs placebo. Psychophysical performance was diminished dose-dependently. Reaction performance was clearly impaired only by the higher dose. The effect on cognitive performance varied, probably as a function of test-specific vigilance optima and the subjects' increase in reactive exertion. However, the subjective well-being of the subjects was impaired throughout.