Rat mothers whose pups have been removed show greater plasma corticosterone elevations when the pups are shocked and returned in a wire-mesh basket than when only the wire basket is returned. For mothers housed with an adult male or virgin female, this differential adrenocorticoid responsiveness between the shocked-pup and empty-basket conditions was not observed unless the adult partner was removed 18 hr before testing. Nonlactating adults housed with pups and mother for up to 2 weeks exhibited corticoid elevations equivalent to the shocked-pup and empty-basket conditions whether tested with the mother or 18 hr after her removal. These results indicate that the differential pituitary-adrenal responsiveness that female rats show to pup cues during lactation cannot be accounted for by the length of the mother's exposure to pups, and, further, that this relative responsiveness during lactation appears to be largely due to the suppression of adrenocorticoid reactivity to a nonsocial environmental disturbance.