Four rhesus monkeys were maintained under a FR 10 schedule of cocaine (0.1 mg/kg) or procaine (0.4 or 1.6 mg/kg) delivery. Haloperidol (0.01-0.08 mg/kg), physostigmine (0.0125-0.1 mg/kg) or saline treatments were administered prior to sessions in which responding was maintained by each of these drugs. Haloperidol produced dose-related increases in the self-administration of cocaine and dose-related decreases in the self-administration of both doses of procaine. Physostigmine produced dose-related decreases in the self-administration of both cocaine and procaine. These results suggest that the reinforcing properties of cocaine are specifically modified by drugs which interact with catecholamines. On the other hand, it seems unlikely that the reinforcing properties of procaine are mediated by the same mechanism. While the results of this experiment indicate that cholinergic mechanisms may not play a major role in mediating the reinforcing properties of either drug, additional studies with other cholinergic agonists and particularly antagonists as well as additional procedures are needed.