The concentration of progesterone in systemic plasma of female rats was proportional to the amount of copulatory stimulation they received when they were in heat. Within 6 hr after mating, females receiving more than 9 intromissions (High Intromission Group) had significantly higher quantities of this hormone in their blood than did females receiving no copulatory stimulation (Control Group). Females in the High Intromission Group showed steadily increasing concentrations of progesterone over the four days following mating, while females receiving a small number of intromissions (Low Intromission Group) or no intromissions (Control Group) did not show this increase. In addition to progesterone, the female rat is known to secrete a second, weaker, gestational hormone: 20α-OH-pregn-4-ene-3-one. On the fourth day after mating, the ratio of progesterone to 20α-OH-pregn-4-ene-3-one in the High Intromission Group was 1.10 compared to 0.43 for the Low Intromission Group and 0.21 for the Control Group. Thus, sufficient stimulation from the male rat's copulatory intromissions initiates a neuro-endocrine reflex in the female; the result of this reflex is an elevated concentration of progesterone in the female soon after coitus, followed by the maintained secretion characteristic of pregnancy.