The projections and odor responses of mammalian olfactory receptor neurons, as well as the physiology of the bulb's principal neurons-the mitral cells (MCs)-are known from studies in slices and anesthetized animals. In behaving rats trained to discriminate between two odors associated with different reinforcers, we examined MC responses following alternated odor-reinforcer pairings. Whereas only 11% of the recorded MCs showed changes in odor-selective firing rate during the odor-sampling phase, 94% of MCs modulated activity during specific behaviors surrounding odor sampling. These cell- and odor-selective responses were not primary sensory responses; rather, they depended (reversibly) on the predictive value of each odor. MC activity thus depends critically on efferent influences linked to the animal's experience and behavior.