Effects of dehydroepiandrosterone-sulphate (DHEA-S) on puerperal lactation and maternal serum prolactin and estradiol levels were examined in 111 cases of normal vaginal delivery without any complication. The amount of lactation in the DHEA-S administrated group was significantly decreased on the second day of the puerperium (p less than 0.05). No significant difference could be found after the third day of the puerperium, or in the total amount of lactation up to the seventh day of the puerperium. There was also no significant difference in the changes in neonatal body weight. No significant change was observed in maternal serum prolactin levels following DHEA-S administration. But the maternal serum estradiol levels were significantly higher at 0, 6, 12, 24 and 36 hours after delivery following the administration of DHEA-S (p less than 0.05 approximately 0.005). These facts suggested that the transient inhibition of lactation by DHEA-S may be caused by the effect on mammary glands of estrogens which were converted from DHEA-S in the placenta.