The exaggerated blood acetaldehyde response that has been reported after ethanol administration to pregnant rats was found to be the beginning of a much larger alteration occurring during lactation. Indeed, at the end of pregnancy, we confirmed a 4-fold increase in the acetaldehyde values above nonpregnant values after an intragastric dose of 3 g/kg ethanol. During gestational days 1 to 17, the levels did not differ. After delivery, the exaggerated acetaldehyde response to ethanol was increased, producing acetaldehyde concentrations 15-fold greater than in nonlactating controls. This response returned to nonpregnant levels with weaning and could be abolished by removing the pups at birth. The intensified response was associated with both an enhanced rate of ethanol oxidation and a decreased low Km aldehyde dehydrogenase activity in liver mitochondria. At the end of pregnancy, measurable concentrations of acetaldehyde were found in umbilical venous blood and fetal blood. However, they amounted to only one-quarter of maternal values whereas ethanol levels were similar. Thus, during late pregnancy and lactation, there is a marked increase in maternal blood acetaldehyde after ethanol intake. In the presence of a normal placenta, however, an acetaldehyde concentration gradient exists between the mother and the fetus.