Pigeons obtained food on a progressive-ratio schedule that required 8 additional responses for each successive reinforcement. The number of responses in the final completed ratio of the session was defined as the breaking point. When cocaine was administered (IM, 5 min presession), the breaking point increased and then decreased as a function of increasing doses (0.3-10 mg/kg). In contrast, across the same range of doses of fenfluramine, the breaking point only decreased. At doses of each drug that decreased the breaking point, the high running rate of responding was interrupted by pauses. At doses of cocaine that increased the breaking point, the running rate was also disrupted, but the disruption was characterized by lower, irregular rates rather than pausing. The increases in breaking point observed at 3 mg/kg of cocaine were no longer seen when fenfluramine was administered at the same time.