The ability of the intravenous administration of mazindol (SaH 42-548) to act as a reinforcer in monkeys previously conditioned to self-administer cocaine was ascertained. Unit dosages i.e. dosage per injection, of 50 and 100 mug/kg resulted in self-administration rates significantly greater than that which occurred with saline. An inverse relationship existed between unit dosage and frequency of self-administration over the unit dosage range 50-200 mug/kg. The total mazindol dosage self-administration per session was however independent of unit dosage. Approximately 2-3 mg/kg was self-administered by each animal during a 4 hr session at each of the 3 unit dosages. This tends to indicate that the 200 mug/kg unit dosage was also reinforcing even though the self-administration rate was similar to that of saline. This study indicates that mazindol can serve as a reinforcer and that the relationship between total session intake, unit dosage, and self-administration frequency of mazindol are similar to these seen with other reinforcing psychomotor stimulant drugs.