Endogenous opioid peptides acting in the medial preoptic area (MPOA) appear to be involved in the regulation of maternal behavior in lactating rats. Moreover, it is known that the density of mu-opiate receptors in the MPOA is elevated during pregnancy, but decreases during lactation. In the first experiment of this study, mu-receptor density in the preoptic area was examined across the periparturitional period on gestation days 18, 20 and 22, at 1 h postpartum and on postpartum days 1 and 12. The effect of pregnancy during lactation on mu-receptor density was also assessed. In addition, plasma hormone concentration of estradiol (E2), progesterone (P), and prolactin (PRL) were determined. While plasma P levels decreased and PRL levels increased prior to parturition, MPOA mu-receptor density remained elevated until 24 h after parturition before declining to reach a level similar to that of ovariectomized control animals. Receptor density was significantly correlated with PRL levels only in gestation day 22 animals, when PRL levels were highest. MPOA mu-receptor density was low at postpartum day 12 whether or not the animal was pregnant. No effects were observed in the adjacent lateral preoptic area in any group. In the second experiment, the effect of hormonal manipulation on preoptic opiate receptor density was examined at various times after removal of Silastic capsules containing P following sustained E2/P exposure. While P levels decreased abruptly following capsule removal, MPOA receptor density declined more gradually. The results are consistent with the hypothesis that a reduction of mu-receptor density occurs in the MPOA following parturition by receptor turnover in the absence of sufficient hormonal stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)