Herein we report four experiments relating to the maternal pheromone of the rat that show (a) preweanling young consume pheromone-containing feces most conspicuously between 14 and 27 days; (b) they consume those feces preferentially; (c) such feces contain high levels of deoxycholic acid; and (d) preweanling young are deficient in deoxycholic acid. The data of these experiments lend support to the hypothesis that in responding to the pheromone the young ingest deoxycholic acid, a steroid believed to promote gut immunocompetence and brain myelination.