The odor of rat litter-mates was experimentally modified from 3-10, 11-18, or 22-29 days of age. Acetophenone or ethyl benzoate were used during the experimental period and the natural odor was present during the other two periods. After living in the natural odor colony for another 2 weeks, Ss were compared on duration of time near each olfactory stimulus animal in an eight hour test. Ss preferred the stimulus animal characterized by the experimental odor with which they were reared. Age of exposure was not a significant factor. These results indicate that the odor of the species is learned, that the intensity of the odor is important, but that age is not a critical variable in the early learning of the species odor in the rat. The odor of rat litter-mates was experimentally modified from 3-10, 11-18, or 22-29 days of age. Acetophenone or ethyl benzoate were used during the experimental period and the natural odor was present during the other two periods. After living in the natural odor colony for another 2 weeks, Ss were compared on duration of time near each olfactory stimulus animal in an eight hour test. Ss preferred the stimulus animal characterized by the experimental odor with which they were reared. Age of exposure was not a significant factor. These results indicate that the odor of the species is learned, that the intensity of the odor is important, but that age is not a critical variable in the early learning of the species odor in the rat.