The present study reveals that the serum prolactin (PRL) levels in early stage of pregnancy exceeded those of the non-pregnant women and rose gradually throughout pregnancy. Within a few days after delivery, PRL declined from maximum values at the end of pregnancy. In non-lactating women serum PRL returned to non-pregnant levels within about one month after delivery, but still remained above non-pregnant levels in lactating women. About one month after delivery serum estradiol was higher in non-lactating than lactating. LH and FSH showed no significant differences between non-lactating and lactating, and were similar to the levels of follicular phase. Therefore the role of PRL on puerperal infertility is thought to have no suppressive effect on the tonic secretion of pituitary gonadotropins and PRL may have a direct effect on the ovary. The excessive response of serum PRL to TRH administration (500 micrograms, i.v.) was observed in lactating women on the 5th postpartum day and during the 28-35 day period, but not during the 90-120 day period. Women given TRH 60 min after the onset of suckling on the 5th postpartum day displayed a relative small increase in serum PRL compared with those after 240 min. The response of PRL to TRH was probably affected by suckling.