Blood concentrations of ethanol and acetaldehyde were determined in suckling rats after a single oral ethanol gavage. These results were compared with the hepatic activities of alcohol and aldehyde dehydrogenase. After intragastric administration of 3 g/kg body weight of ethanol, ethanol concentrations were much higher in suckling rats than in adult animals, especially at 90, 120, and 180 min after its administration. In addition, acetaldehyde concentrations were undetectable in suckling rats as opposed to adult rats, in whom micromolar concentrations were detected. Thus, 5- to 30-day-old rats seem to have a limited capacity for in vivo ethanol metabolism. The analysis of hepatic alcohol dehydrogenase activity revealed that it was very low at birth and it increased progressively with time attaining adult levels after 20 days of life. The alcohol dehydrogenase activity present in the liver of suckling rats presented similar Km values and sensitivity to pyrazole as adult rat liver. Thus, the pattern of in vivo ethanol elimination during the suckling period is not explained by hepatic alcohol dehydrogenase activity. Whether that diminished ethanol metabolism is due to slower intestinal ethanol absorption, different ethanol distribution in the body, or diminished hepatic capacity for NADH reoxidation remains to be studied. At birth, hepatic aldehyde dehydrogenase activity was low and it increased reaching adult levels during the suckling period. Adult levels for the component of low Km were attained earlier than for the component of high Km. The low affinity hepatic aldehyde dehydrogenase component in the newborn was different from that in the adult as assessed by kinetic studies and by its sensitivity to disulfiram.(ABSTRACT TRUNCATED AT 250 WORDS)