In summary, studies from our and other laboratories strongly suggest that placental lactogen has direct effects on fetal growth and metabolism as well as on maternal metabolism. Prolactin may be important in the regulation of water and ion transport across the amnion, the production of surfactant by the fetal lung, and the immune response during pregnancy. A summary of the postulated effects of placental lactogen on maternal and fetal physiology is depicted in Figure 9 and a summary of the postulated effects of prolactin during pregnancy is shown in Table 6. Undoubtedly, future studies of the physiology of placental lactogen and prolactin will uncover new functions for these hormones during gestation.