The disposition of ethanol and its metabolite, acetaldehyde, and the activity of alcohol dehydrogenase (ADH) and aldehyde dehydrogenases (ALDH) were determined in the third-trimester pregnant guinea pig following single and 7-day oral administration of ethanol (0.5 g X kg maternal body weight-1 X day-1). Animals were killed at each of selected times after the single and seventh ethanol dose. For both ethanol dosage regimens, the maternal and fetal blood and brain ethanol concentrations were virtually identical during the elimination phase of the time-course study. There was initial slow transfer of ethanol into amniotic fluid, followed by significantly higher ethanol concentration in amniotic fluid relative to maternal and fetal blood during the elimination phase. Acetaldehyde was measurable in maternal blood, maternal brain, and fetal brain at concentrations that were low and variable. For both ethanol dosage regimens, ADH activity was measurable only in maternal liver. Low Km ALDH activity was measurable only in maternal liver and fetal liver. High Km ALDH was measurable in maternal liver, fetal liver, and placenta and was significantly greater in maternal liver. The data indicate that there is bidirectional placental transfer of ethanol in the maternal-fetal unit; the elimination of ethanol from the maternal and fetal compartments is regulated by maternal hepatic biotransformation involving ADH; the amniotic fluid is a reservoir for ethanol in utero; the low Km ALDH in fetal liver protects the fetus from ethanol-derived acetaldehyde in the maternal circulation; and short-term maternal administration of once-daily, low-dose ethanol does not produce major changes in ethanol disposition and the activity of the enzymes involved in ethanol biotransformation.