Changes of serum LH (HCG) and FSH, and their response to LH-RH during pregnancy and puerperium were studied by radioimmunoassay. There was little change in serum FSH level due to progression of pregnancy, and the serum FSH level of pregnant women was almost unchanged after injection of LH-RH. Serum LH (HCG) which decreased rapidly after delivery returned at about 15 days after parturition, and FSH which remained at a low level during pregnancy returned to the levels in the follicular phase of the normal women at about 20 days after delivery. The alteration in the response to LH-RH in puerperal period was as follows: First FSH began to respond about 10 days after delivery, and one month after parturition both LH and FSH showed remarkable response in comparison with that in the normal non-pregnant period. Beyond the second postpartal month, the response of FSH returned to approximately the same level as that in the normal non-pregnant period, but the response of LH was still marked when compared with that in the non-pregnant period. There were no differences in the response to LH-RH between the nursing and non-nursing women.