Four pregnancies after renal transplantation, during which estrogen metabolism was monitored, were reviewed. Urinary estrogen levels were obtained in all four pregnancies, and all values were low. This phenomenon has been ascribed to suppression via the fetal pituitary of the fetal adrenal glands by exogenously administered corticosteroids. However, definitive data substantiating this theory are lacking. Serum or amniotic fluid estrogen levels were measured concomitantly, and in each instance the levels were below normal for the period of gestation. This finding suggests that the mechanism producing low levels of urinary estrogens is related to the suppressed synthesis of estrogen rather than to altered renal excretion of estrogen.