The pituitary-adrenal system plays an important role in the initiation and maintenance of lactation in rats, with the suckling stimulus itself inducing the release of ACTH. The recent finding of a reduced plasma corticosterone response to a variety of noxious stimuli during lactation led us to further investigate the activity of this system in lactating (L) rats, compared with post-parturient non-lactating ontrols (NL). Plasma and adrenal corticosterone and plasma ACTH were measured, the latter with a mouse bioassay. Over a 24 h period (12 L:12 D), basal concentrations of plasma corticosterone were elevated in L females, only at those times when the NL basal concentrations were at the trough in the diurnal cycle. At all times, the plasma corticosterone increase 15 min after ether was significantly lower (by a mean of 55 %) in L rats than in NL rats. The elevations in plasma corticosterone after ether were higher for all rats during the day. The night-day difference in response to ether was greater in L rats than in NL rats. Although morning basal levels were not significantly elevated in L females deprived of their litters for 24 h, in these females, as in continually lactating rats, stress concentrations of plasma ACTH (2–4 min after ether) were one third that of the NL controls. Finally, after dexamethasone treatment, the corticosterone output to exogenous ACTH was lower in the plasma and higher in the adrenal in L rats. Thus, pituitary-adrenal activity is altered in a variety of ways during lactation.