It has been demonstrated that many conditional responses to a variety of drugs are opposite in direction to the unconditional effects of the drug, and the conditioning analysis of morphine tolerance emphasizes the fact that subjects with a history of morphine administration display morphine-compensatory conditional responses when confronted with the usual administration procedure but without the drug. Thus, when the drug is presented in the context of the usual administration cues, these conditional morphine-compensatory responses would be expected to attenuate the drug-induced unconditional responses, thereby decreasing the observed response to the drug. Research has been summarized which supports this compensatory conditioning model of tolerance by demonstrating that the display of tolerance is specific to the environment in which the drug has been previously administered. Further evidence supporting this theory of tolerance has been provided by studies establishing that extinction, partial reinforcement, and latent inhibition--non-pharmacological manipulations known to be effective in generally affecting the display of conditional responses--similarly affect the display of morphine tolerance. Additional research has suggested many parallels between learning and morphine tolerance: Both processes exhibit great retention, both are disrupted by electroconvulsive shock and frontal cortical stimulation, both are retarded by inhibitors of protein synthesis, and both are facilitated by antagonists of these metabolic inhibitors.