Ovaries from pregnant and postpartum Sprague-Dawley rats were examined for content of immunoreactive beta-endorphin by radioimmunoassay, and for its localization by the peroxidase-antiperoxidase technique. In addition, the molecular forms of beta-endorphin immunoreactivity were separated by gel chromatography and reverse-phase high performance liquid chromatography (RP-HPLC). Ovaries from rats early in pregnancy showed intense granular cytoplasmic staining of luteal cells, with an even distribution of granular material throughout the cytoplasm. By middle to late pregnancy the staining pattern was changed, with immunoreactive material showing a less granular and unevenly distributed staining pattern and with some areas of the cytoplasm totally devoid of immunoreactive material. The concentrations of immunoreactive beta-endorphin measured during pregnancy were significantly lower than levels in mature non-pregnant rat ovary. The ovarian concentration of immunoreactive beta-endorphin fell progressively during pregnancy and early lactation, returning to normal cyclic rat levels at 20 days post partum. The ovarian concentration of beta-endorphin-like material was lowest at 6 days post partum (0.53 +/- 0.08 ng/g wet weight; mean +/- S.E.M.), representing approximately 10% of the concentration found in pooled ovaries from randomly cyclic adult rats. Gel chromatography revealed only a single peak of immunoreactive beta-endorphin, co-eluting with 3.5 kD molecular weight ovine beta-endorphin (1-31). This contrasts with gel profiles of adult cyclic rat ovary, where large molecular weight species pro-opiomelanocortin (31 kD) and beta-lipotrophin (11.5 kD) are also present. On RP-HPLC the predominant species of low molecular weight immunoreactive material co-eluted with beta-endorphin(1-31).(ABSTRACT TRUNCATED AT 250 WORDS)