Changes in opioid concentrations in brain and plasma as well as opioid activity have been reported to occur as a function of pregnancy and lactation in rats. The present study examines the status and steroidal regulation of the endogenous opioid, beta-endorphin, in the behaviorally and neuroendocrinologically important preoptic area (POA) and hypothalamus, and in the plasma of pregnant and nonpregnant rats. In the first study, concentrations of beta-endorphin-like immunoreactivity (β-EP-LI-Ir) in POA and hypothalamic tissues as well as in plasma were measured throughout gestation in rats. β-EP-LI-Ir concentrations in the POA were significantly higher in rats from day 6 to 18 of gestation than in nonpregnant, diestrous females. β-EP-LI-Ir concentrations in the POA declined significantly between day 18 and 22 of gestation. Changes in hypothalamic β-EP-LI-Ir concentrations were not detected either as a function of pregnancy or during pregnancy, while plasma β-EP-LI-Ir concentrations declined gradually from day 6 to 18 of pregnancy and then increased significantly prepartum (day 22 of gestation). In the second study, the effects of 2 weeks of exposure to pregnancy levels of progesterone and estradiol on brain and plasma β-EP-LI-Ir were measured. Exposure to the combination of progesterone and estradiol (administered subcutaneously via Silastic capsule implants) resulted in a significant increase in β-EP-LI-Ir concentrations in the POA, but did not affect β-EP-LI-Ir concentrations in either the hypothalamus or plasma. In the final study, the effects of estradiol exposure on β-EP-LI-Ir concentrations in brain were evaluated. Exposure to a high, but not low, dose of estradiol resulted in a significant reduction in β-EP-LI-Ir concentrations in the hypothalamus and a reduction, although nonsignificant, in the POA. These findings indicate that the increase in β-EP-LI-Ir concentrations found in the POA from day 6 to 18 of pregnancy results from exposure to high circulating levels of progesterone in the presence of estradiol, and that the prepartum decline in POA β-EP-LI-Ir concentrations may be stimulated by the prepartum shift in steroidal environment from one of progesterone dominance to one of estradiol dominance. The significance of these changes in β-EP-LI-Ir concentrations are discussed in terms of opiate regulation of maternal behavior and neuroendocrine function during the periparturit