delta9-Tetrahydrocannabinol: tolerance after noncontingent exposure in rats.

Rats pretreated daily with delta9-tetrahydrocannabinol (delta9-THC, 8 mg/kg) during 14 days showed tolerance to THC-induced hypothermia and depression of CAR acquisition in a shuttle-box. The noncontingent exposure of THC also produced tolerance to spontaneous (unlearned) behaviors as measured in a open-field test. The data suggest no essential role for learned tolerance in the sense that animals have to to learn to perform under the influence of THC for several of these behaviors.