Under unsatisfactory environmental conditions, such as the mother's illness, placental insufficiency, PROM, etc., the fetus must make the transition to extrauterine life from an unfortunate intrauterine existence. The adaptation process of the fetus to extrauterine environment in these cases is extreme importance. In our present investigation, we studied the developmental pattern of the functions of each fetal organ and the specific aspects of fetal metabolism. Using the brush border technique, we clarified that the fetal absorptional function of intestine developed from the 24th week of gestation and reached adult level in 32nd gestational weeks. The fetal renal reabsorption function developed from the 30th week of gestation and reached adult level in 36 gestational weeks. The liver function (enzymatic activity, etc.) remained low during the entire fetal life and developed gradually after birth. Taurine plays an important metabolical role and has a specific transport system during fetal life. The coagulation of vitamin K-dependent factors and activity remained low level at 27-31st gestational week, reaching full term level after 30 days. The switching of HbF to HbA was delayed more significantly in premature infants than in full term infants, but this delay of switching is effectively corrected by the increase of 2.3 DPG in premature infants. Immune response remained immature even in the full term fetus, but the non-specific immune response (NK activity, LAK activity, etc.) developed in the early fetal period and showed characteristic activity. The steroids which originated from the fetal-placental unit related intimately to the continuity of pregnancy and fetal development.