Twenty-one-day-old male weanling rats were started on a chronic exposure regimen of 0, 100, or 300 ppm lead acetate in distilled water. They were trained to press a lever for food reward at 55 days of age. Subsequently, a schedule of differential reinforcement of minimum response duration was imposed, in which only lever press durations greater than or equal to a required duration were followed by food presentation. Under these conditions, response durations of control animals approximated schedule requirements; lead treatment shortened response durations. Treated rats also paused for longer periods before initiating responses. Differences between the treated groups were not apparent. Individual differences were observed in susceptibility to lead-induced behavioral changes. Some treated animals tended to show greater session-to-session variability than did controls. Even when a tone was added to signal that the required duration had elapsed, certain treated rats failed to show modal response duration values consistent with the schedule requirement. Failure to improve performance of lead-treated rats by external stimulus control has not previously been reported.