Serum alpha-lactalbumin was monitored throughout pregnancy in twelve women and in a separate group of nineteen women during the first 3 months postpartum. During pregnancy alpha-lactalbumin rose significantly until the mid trimester (P less than 0.001). From then until term, concentrations remained stable. Concentrations during labour were significantly higher (P less than 0.01) than those seen at term. alpha-lactalbumin, 17 beta-oestradiol and progesterone concentrations behaved similarly during the first week of the puerperium in both lactating (n = 10) and non-lactating (n = 9) subjects. A large surge of alpha-lactalbumin closely followed the clearance of high circulating concentrations of sex steroids in both groups. Prolactin concentrations were significantly greater (P less than 0.02) in lactating subjects by the third postpartum day. By the third postpartum week alpha-lactalbumin concentrations in lactating subjects had stabilized at labour levels in a milieu of high prolactin levels and depressed production on 17 beta-oestradiol and progesterone. Conversely, in non-lactating subjects alpha-lactalbumin concentrations fell, as did prolactin, coincidental with a rise in 17 beta-oestradiol, progesterone concentrations remaining barely detectable. The apparent control mechanisms for human alpha-lactalbumin secretion and thus, lactation, are discussed in the light of the data presented.