Concentrations of immunoreactive inhibin in serum samples collected daily from six adult stumptailed female macaques during normal menstrual cycles were measured with a heterologous radioimmunoassay. Serum inhibin concentrations were low during the follicular phase of the cycle. After ovulation they began to rise, reaching a plateau between 8 and 11 days, before falling in parallel with the decline in luteal progesterone secretion. The dependence of the inhibin secretion by the corpus lutem on pituitary gonadotrophins was investigated by the administration of an LHRH antagonist [N-Ac-D-Nal(2)1,D-pCl-Phe2,D-Trp3,D-hArg(Et2)6,D-Ala10]LHRH once daily for 3 days beginning on day 8 of the luteal phase in six macaques. LHRH antagonist treatment markedly suppressed serum levels of inhibin and progesterone and these remained at the level found in the follicular phase for the remainder of the luteal phase. These results show that inhibin in the macaque is secreted into the peripheral blood almost exclusively during the luteal phase, being highest when FSH is at its nadir. Suppression of serum inhibin concentrations during the luteal phase by LHRH antagonist suggests that its secretion is integrated with the LH control of the corpus luteum.