Abstract (1) Interactions of estrogen and progesterone with each other and with the neural tissues that regulate sexual behavior in female guinea pigs were studied. (2) Long-acting preparations of estradiol-17β (E 2) were more effective in facilitation of lordosis than other estrogens. (3) A major behavior-facilitating site of action of E 2 is in the medial basal hypothalamus. (4) E 2 is selectively taken up by a saturable receptor system in fractions of hypothalamic tissue. (5) Although early effects of E 2 on neural tissues that mediate lordosis can be mimicked by certain anti-estrogens, anti-estrogens do not mimic long-term effects of E 2 that are required for optimum expression of lordosis behavior. (6) Progesterone (P) is required in extremely small quantities for facilitation of lordosis behavior in estrogen-primed female guinea pigs. (7) Facilitation of sexual behavior is a short-term effect of P that is mediated by the medial basal hypothalamus. (8) P inhibits the expression of lordosis behavior via a mechanism that is represented in the midbrain. (9) This inhibitory action of P has a longer latency than the facilitatory action of P. (10) Prolonged residence of E 2 in the hypothalamus temporarily favors the expression of short-term facilitatory actions of P on lordosis. (11) The long-term lordosis-antagonizing effects of P are not due to inhibition of E 2 uptake in the hypothalamus. (12) Inhibitory effects of P on lordosis may not depend on neural cells that have the ability to concentrate E 2 in their nuclei.