Delta-9-Tetrahydrocannabinol (THC) was administered to albino rats with extensive experience in free-operant (Sidman) lever-press shock avoidance. Dosing (30 mg/kg intragastrically) continued once daily, 3 hr before testing, for 1 to 6 weeks. Significant changes were noted in the response rates of several animals, but both the magnitude and direction of these were highly variable. However, shock rates were reliably elevated by THC, but complete tolerance was observed within 6 sessions. In several rats this was followed by sessions with significantly lower shock rates than the predrug baseline. These rats continued to perform at this level of proficiency until THC was discontinued, at which point the baseline was reacquired. These data emphasize that an important determinant of tolerance to a drug effect is the consequence of the effect for the organism.