Rats treated chronically with delta 9-tetrahydrocannabinol (THC, daily oral dose 20 mg/kg) were examined for residual effects on a variety of behaviors following a 1-4-month drug-free period. Learning a 12-arm radial maze and a differential reinforcement of low-rate responding (DRL-20) task was significantly retarded in THC-treated animals, although performance reached control levels by the end of testing. Learning two-way shuttle box avoidance was slightly facilitated in the drug-treated subjects. In open field tests THC-treated rats displayed an initial hypoactivity, followed by hyperactivity, but these changes were not significant. Most of the effects of THC resemble, but are weaker than those of chronic treatment with cannabis extract in a dose containing the same amount of THC. The findings are discussed in terms of the role of other constituents of cannabis that may add to, or potentiate the effects of THC itself.