Immunoreactive beta-endorphin (IR-BE) levels in the plasma, anterior pituitary (AP), the neurointermediate lobe of the pituitary (NIL), and the hypothalamus were determined in castrated female rats and castrated female rats treated with estradiol benzoate (estrogen), after exposure to acute (once for 45 min) or chronic (45 min each day for 15 consecutive days) immobilization stress. Acute and chronic stress increased plasma levels of IR-BE to the same extent in castrated female rats and castrated female rats treated with estrogen. In castrated female rats, acute stress produced an increase in the concentration of IR-BE in the AP, which was attenuated by the administration of estrogen. Although IR-BE in the NIL was not influenced by acute stress in castrated animals, exposure to acute stress resulted in an elevation in IR-BE levels in the NIL of rats given estrogen. Chronic stress did not affect the concentration of IR-BE in the AP of castrated females or castrated females treated with estrogen. Chronic stress did, however, increase the concentration of IR-BE in the NIL of castrated animals. This affect of stress on IR-BE levels in the NIL was potentiated by estrogen administration. IR-BE levels in the hypothalamus were reduced by estrogen and were not affected by acute or chronic stress, regardless of the gonadal steroid environment. As determined by column chromatography, administration of estrogen, as well as subjection to chronic stress, promoted the processing of the proopiomelanocortin precursor to form beta-lipotropin rather than beta-endorphin in the AP. By these methods, the only immunoreactivity detected in the NIL and the hypothalamus was beta-endorphin. These data indicate that IR-BE levels in the plasma, the AP, and the NIL of female rats are affected by immobilization stress and that estrogen modulates the effects of acute immobilization stress on IR-BE levels in the AP and the NIL and the effects of chronic immobilization stress on the levels of IR-BE in the NIL.