beta-Endorphin-like immunoreactivity (beta-EP-LI) was measured by RIA in plasma collected from pituitary portal vessels of rats at various times in the estrous cycle and after ovariectomy. There were no appreciable differences between the mean beta-EP-LI concentration either in the afternoon (1500-1700 h) of estrus (4.1 +/- 1.5 ng/ml) or on diestrous days 1 (4.1 +/- 1.3 ng/ml) or 2 (5.4 +/- 1.5 ng/ml). The concentration increased slightly (6.7 +/- 1.3 ng/ml) but not significantly in the early afternoon (1400-1500 h) of proestrus. The concentration of beta-EP-LI then fell to 0.5 +/- 0.1 ng/ml at 1700-1800 h on proestrus, a level significantly lower than at any other time of the cycle. Portal plasma beta-EP-LI was also low (1.9 +/- 0.5 ng/ml) in animals ovariectomized for 3 weeks. After gel filtration of the portal plasma extracts, most of the beta-EP-LI eluted in the same position as synthetic beta-EP. Dilution of portal plasma produced a displacement curve parallel to that of beta-EP and hypothalamic extract. These results indicate that the secretion of hypothalamic beta-EP into the blood of pituitary portal vessels changes during the estrous cycle, possibly due to cyclic changes in sex steroids.