Pseudopregnancy inhibited plasma i corticosterone response to stress. The production of corticosterone by adrenal slices in vitro was also decreased; no changes in adrenal 5α-reductase activity were observed. Secretion of corticosterone in the adrenal venous blood was decreased as well, without changes in dehydrocorticosterone or tetrahydrocorticosterone secretion rates. Treatment with ACTH increased the secretion rates of corticosterone to the range observed in control rats. Steady state secretion and metabolism of corticosterone were unchanged. The data indicate that the diminished responses in plasma concentrations, in vitro production, and secretion rates of corticosterone obtained in pseudopregnant rats after stress, are secondary to changes on the release of ACTH from the pituitary gland rather than to adrenal steroid secretion per se or handling of corticosterone by the liver. The expected rise in plasma prolactin observed in cycling rats after stress was absent in pseudopregnant rats. The data suggest that an inhibition of the prolactin release mechanisms similar to that observed with the release of ACTH may have occurred after stress. The dynamics involved in the changes affecting the release of pituitary hormones in pseudopregnant rats are still unclear