compound stimuli was investigated in opossums using the Rno R (go - no go) differentiation. Each compound stimulus consisted of two factors: quality and location. Each correct response performed to the conditioned positive, or 'safe' stimulus, was rewarded by food and never punished. Each incorrect response performed to the conditioned negative, or 'warning' stimulus, was punished by an electric shock. In subsequent testing, each opossum proved to use only the-quality cues to solve the task even though later testing showed them capable of using the location cues. Thus, the rule discovered in higher mammals, that the efficacy of auditory stimuli in differentiation depends on the perceptual ability of the animal as well as the type of the behavioral response with which the animal is confronted, may be extended to neurologically primitive mammals and also to a joint conditioned approach-avoidance method.