Aldehyde dehydrogenase (ALDH) isozyme I deficiency in hair root samples from 105 healthy individuals and 72 alcoholics was determined using isoelectric focusing. From these individuals, 12 male alcoholics (2 with ALDH isozyme I deficiency and 10 normal) and 45 healthy controls (18 with ALDH isozyme I deficiency and 27 normal) were investigated for their blood ethanol and acetaldehyde levels by gas chromatography after an acute dose of alcohol (0.5 g ethanol/kg body wt.). Peak blood ethanol values of about 10 mmol/l were attained after 1 hour both in alcoholics and normal controls irrespective of their ALDH type. There was no significant difference in the blood ethanol level during the 5 hr post-drinking period in both the groups. Peak blood acetaldehyde concentration was significantly higher in healthy controls and alcoholics deficient in ALDH isozyme I after alcohol drinking (about 30 micro-mmol/l) than in individuals with normal ALDH isozyme I (3 micro-mmol/l). However, no significant difference in blood acetaldehyde was observed between alcoholics and controls.