The disposition of ethanol and acetaldehyde in maternal venous blood and amniotic fluid was studied over 3.5 hours following the ingestion of 0.3 gm/kg of ethanol by six healthy pregnant women at 16 to 18 weeks' gestation. There was a time lag in the appearance of ethanol in amniotic fluid compared with its appearance in maternal venous blood. The maximum maternal venous blood ethanol concentration was twice the maximum amniotic fluid ethanol concentration and occurred at an earlier time. At 3.5 hours, ethanol was present in amniotic fluid, whereas there was virtually no measurable ethanol in maternal venous blood. The rate of ethanol elimination from amniotic fluid was about half the elimination rate from maternal venous blood. Acetaldehyde was present in the maternal venous blood of four subjects, and the maximum acetaldehyde concentration occurred at about the same time as the maximum maternal venous blood ethanol concentration. Acetaldehyde was found in the amniotic fluid of one of these four subjects, and the acetaldehyde concentration was greater in amniotic fluid than in maternal venous blood. It is proposed that the amniotic fluid may act as a reservoir for ethanol following maternal ingestion such that the fetus would be exposed to ethanol for a longer time period than would be predicted by the maternal venous blood ethanol concentration.