Pretreatment by a psychoactive drug can greatly attenuate the conditioning of gustatory avoidance by that drug. Although such findings have been interpreted in terms of tolerance, alternative explanations are possible. In a series of experiments, it was found that pretreatments with morphine or amphetamine massed at 24-h intervals were no more effective in attenuating conditioning than pretreatments spaced at 120-h intervals, but pretreatment with morphine provided more persistent protection against subsequent conditioning by itself than did amphetamine in a comparable previous experiment. The similarity of massed and spaced pretreatment effects can be interpreted without appealing to tolerance as a factor, but the greater persistence of morphine pretreatment implicates tolerance as a mechanism.