Dose-response curves were determined for the effects of morphine (0.3–10 mg/kg) and pentazocine (1.0–30 mg/kg) on continuous avoidance responding in the rat. Each dose of morphine was retested following 3 days of morphine administration. The pentazocine curve was redetermined after 3 days of treatment with either pentazocine or morphine. Upon initial testing, morphine and pentazocine both generated biphasic dose-response curves. Graded increases in response rate were produced by 0.3–3.0 mg/kg of morphine and by 1.0–10 mg/kg of pentacozine; behavior was disrupted by 10 mg/kg of morphine and by 30 mg/kg of pentazocine. The stimulant effects of the lower doses of morphine and pentazocine were markedly reduced by 3 days of drug treatment; the disruptive effect of the highest dose of each drug was similarly attenuated. These findings show that tolerance can develop to a stimulant component of action of morphine and pentazocine in the rat. The development of pentazocine cross-tolerance to morphine provides additional support for the view that a common mechanism mediates the effects of morphine and pentazocine on avoidance behavior.