The effects of estradiol-17β (E2) and progesterone (P), administered sc in silastic capsules, on the serum levels of LH and FSH were studied over four days in two groups of intact rhesus monkeys on days 3 to 9 of the intermenstrual period and in ovariectomized (ovx) animals. In the intact females (N = 5, Group 1), simultaneous LH and FSH surges occurred approximately 48 h after E2 treatment was begun (serum E2 levels of 200 to 400 pg/ml). With this same E2 treatment was accompanied by P (serum levels of 4 to 6 ng/ml, Group 2), LH release was blocked in 6 of 6 animals. In ovx animals (N = 9, Group 3), E2 alone produced an LH surge at 72 h (P>0.05 Group 1 vs 3), and a slight release of FSH with LH in 6 of the 9 animals. P administered simultaneously with E2 to these same ovx animals (Group 4) advanced (P2 (Group 5). Again the LH surge was advanced (P<0.01 Group 3 vs 5) to approximately 30 h and under these circumstances was accompanied by a distinct release of FSH.