Five squirrel monkeys were trained to stable levels of performance on fixed ratio schedules of food reinforcement. N,N-dimethyltryptamine (DMT) was then injected intramuscularly prior to fixed ratio testing and the behavioral effects observed. The determination of dose-response relationships with doses of 0.0, 5.4 and 10.8 Μmoles/kg indicated that latencies to initiate responding were an increasing function of the dose, while overall response rates, as measured from the first to the last response of the session were unaffected. When a dose of DMT which initially disrupted operant behavior was administered once daily for 36 to 38 consecutive days, tolerance was not observed to occur.