OBJECTIVE To establish a gestational age reference range for fetal plasma aldosterone concentration and to determine whether anemia, growth retardation, or hydrops are associated with abnormal levels. METHODS Aldosterone concentration was measured in umbilical venous blood obtained by funipuncture from pregnancies complicated by red blood cell isoimmunization (n = 17), fetal growth retardation (n = 8), and nonimmune hydrops fetalis (n = 17). Values were compared to reference ranges constructed from the study of samples obtained by funipuncture or at elective cesarean delivery from 40 essentially normal fetuses and maternal blood from 33 uncomplicated pregnancies. RESULTS In the control group, the fetal plasma aldosterone concentration increased linearly with gestation, reaching adult levels at term. In nonimmune hydrops and red blood cell isoimmunization, the aldosterone concentration was increased. In the growth-retarded fetuses, the levels were not significantly different from normals. CONCLUSIONS The gestational age-related increase in fetal plasma aldosterone concentration presumably reflects maturation of adrenal function. In certain abnormal pregnancies, fetal aldosterone concentrations increase.