Rats were trained to discriminate a dose of 1.75 mg/kg phencyclidine (PCP) from saline. During substitution tests, both PCP (0.3-10.0 mg/kg) and the non-competitive NMDA antagonist, MK-801 (0.01-0.3 mg/kg) substituted for the PCP stimulus in a dose-dependent manner. In contrast, delta 9-tetrahydrocannabinol (delta 9-THC, 0.1-5.6 mg/kg) and delta 8-tetrahydrocannabinol (delta 8-THC, 0.3-5.6 mg/kg) failed to substitute for the PCP stimulus, up to doses that substantially decreased rate of responding. However, both delta 9-THC and delta 8-THC partially attenuated the discriminative stimulus effects of the PCP training dose. Furthermore, a dose of 3.0 mg/kg delta 9-THC shifted the PCP dose-effect curve for discriminative stimulus effects to the right and shifted the PCP dose-effect curve for rate of responding to the left. The attenuation of the PCP stimulus by delta 9-THC lacked a strong dose-dependent relationship and was observed both at doses which did not alter rate of responding, as well as at doses which substantially decreased rate. In contrast to the effects observed with delta 9-THC and delta 8-THC, morphine, d-amphetamine and chlordiazepoxide failed to attenuate the discriminative stimulus effects of PCP, even at doses that markedly decreased rate of responding. The present findings suggest that delta 9-THC and delta 8-THC alter the discriminative stimulus effects of PCP in a pharmacologically specific manner.