Five normal women using barrier forms of contraception has serum samples drawn every 2 days during control menstrual cycles. Relaxin was not detectable in their serum. All five subjects then received 2500 IU hCG im every 2 days beginning 8--10 days after presumed ovulation. All had immunoreactive relaxin detectable in serum 2--6 days after the first hCG injection. Relaxin rose to pregnancy levels (range, 0.26--0.62 ng/ml; mean, 0.38 +/- 0.17 ng/ml) by cycle day 31 and then declined before menstruation. Progesterone levels and cycle lengths of relaxin-secreting cycles were significantly higher (P less than 0.05) than control cycles. Three of the five subjects subsequently received 2500 IU hCG im every 2 days commencing 2--3 days after presumed ovulation until menstruation. Relaxin was not detectable in these cycles. hCG can induce relaxin secretion in nonpregnant women. A hCG stimulus which can induce relaxin secretion from an 8- to 10-day-old corpus luteum cannot induce relaxin secretion in a 2- to 3-day-old corpus luteum, suggesting that the age of the human corpus luteum of the menstrual cycle is an important determinant of responsiveness to hCG.