l-Dgr9-trans-Tetrahydrocannabinol in Pigeons: Tolerance to the Behavioral Effects.

Delta(9)-Tetrahydrocannabinol was injected daily, in increasing doses, in pigeons under a multiple schedule of food presentation. Within a week, a dose that initially abolished responding completely was without effect. This dose was gradually increased to 20 times its original value without disrupting the behavior. No withdrawal syndrome was detected when the cannabinol was discontinued.