Alcohol and acetaldehyde blood levels were measured in chronic alcoholic pregnant rats and their fetuses at 15, 19 and 21 days of gestation. Similar ethanol concentrations were found in fetal and maternal blood in all gestation periods studied, however levels in amniotic fluid were higher than in mother's blood, especially in the early stages of gestation. Acetaldehyde concentrations were always lower in fetal than in maternal blood although increasing throughout gestation. The levels in fetal blood and amniotic fluid compared to maternal blood, were ca. 40, 50 and 70% at 15, 19 and 21 days of gestation, respectively; those for the placenta and fetal tissues were lower, i.e., 25, 40 and 50%. Similar alcohol and acetaldehyde ratios (fetus/mother's concentration) were obtained when pregnant non-alcoholic rats were administered cyanamide and ethanol (2 g/kg) at 11, 15, 19 and 21 days of gestation. These results demonstrate that ethanol freely crosses the placental barrier, but there is a concentration gradient of acetaldehyde between mother and fetus which varies with gestation age.