Delays of reinforcement were imposed while three rhesus monkeys intravenously self-administered either cocaine or procaine to determine whether the magnitude of delay-induced reductions in infusion frequency were dose- or drug-dependent. Drug-maintained baselines demonstrating the greatest resistance to change under delay conditions might be conceptualized as involving the greatest strength of responding and include drug reinforcers with the greatest efficacy. Under baseline conditions, cocaine or procaine was infused contingent upon lever pressing according to fixed ratio 10 schedules during daily, 1-h experimental sessions. Under delay conditions, a 120-s delay of reinforcement was inserted between the completion of each fixed ratio contingency and infusion delivery. Tests under baseline and delay conditions alternated until several doses of cocaine (3, 10, 33, 100, and 300 micrograms/kg), and subsequently, of procaine (33, 100, 300, 1000, and 3000 micrograms/kg) were tested. The results indicated that during both baseline and delay conditions increases in dose of either drug resulted in increases followed by decreases in the number of infusions obtained. Introducing delays of reinforcement resulted in decreases in the number of infusions obtained at a given dose. Generally, however, when reinforcement delays were imposed the decreases in infusion number, as a percentage of baseline number, were less at the higher doses of both drugs. The doses of procaine which were maximally effective as reinforcers (i.e. during which infusion number was minimally reduced by delay) were as effective as the maximally effective doses of cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)