In a continuing effort to elucidate the processes underlying in primates, the concentrations of estradiol, estrone and progesterone were measured in ovarian tissues and in ovarian vein plasma through the luteal phase of the menstrual cycle of the rhesus monkey. The concentration of progesterone in corpora lutea collected 4-6 days after the preovulatory LH peak (early luteal phase) was more than twice that found in those collected 8-13 days after the LH surge (late luteal phase) while estradiol and estrone concentrations in the latter had increased 4-fold. These changes in luteal steroid concentrations were paralleled by a striking increase in the concentration of estrone in the ipsilateral ovarian vein. Estrone predominated in the venous effluent of the ovary beaing the corpus luteum while estradiol concentrations were similar in both ovarian veins suggesting that estrone is the principal estrogen released by the corpus luteum. The results of this study are consonant with the hypothesis that estrogen produced by the corpus luteum is the physiologic luteolsin in the rhesus monkey.