Serum concentrations of prolactin, LH, estradiol, and progesterone were measured daily throughout one or more menstrual cycles in 7 rhesus monkeys. Prolactin concentrations at 0800 - 0900 h during 13 menstrual cycles averaged 10.7 ± 0.2 ng/ml (± SE). During none of the cycles did a significant rise in serum prolactin occur to coincide with preovulatory surges in estradiol (368 ± 29 pg/ml) and LH (4.2 ± 0.6 ng/ml). However the mean prolactin level during the follicular phase of all cycles (10.1 ± 0.4 ng/ml) was slightly (P>0.05) lower than that during the luteal phase (11.2± 0.4 ng/ml) when progesterone levels were elevated.