Concentrations of beta-endorphin were measured in the venous effluent of the hypothalamus (hypophyseal portal blood) at various phases of the menstrual cycle and after ovariectomy in rhesus and pigtailed monkeys. In the rhesus, beta-endorphin concentrations were high during the mid- to late follicular phase [737 +/- 256 pg/ml (mean +/- SE)] and the luteal phase (1675 +/- 1108) of the menstrual cycle, but were undetectable (less than 133) at menstruation. Concentrations were also high in pigtailed monkeys during stages of the menstrual cycle other than at menstruation (4870 +/- 1090 pg/ml), but undetectable (less than 133) 4--12 months after ovariectomy. These results indicate that beta-endorphin concentrations in hypophyseal portal blood are related to menstrual cycle events, probably changes in ovarian steroids; this in turn suggests that beta-endorphin may participate in the ovarian feedback regulation of gonadotropin secretion.