1 The disposition kinetics of ethanol and its toxic metabolite acetaldehyde were investigated in 10 healthy male volunteers who ingested 0.25 g kg-' ethanol after an overnight fast. This dose of ethanol was given 2 h after they swallowed a tablet of either calcium carbimide CC (50 mg), a potent inhibitor of low Km aldehyde dehydrogenase (ALDH), or placebo according to a single-blind crossover design. 2 The pulmonary blood concentrations of ethanol and acetaldehyde were estimated indirectly by means of a gas chromatographic method modified for analysis of end-expired breath. This non-invasive sampling technique allowed replicate determinations at 15 min intervals.3 The distribution volume of ethanol (V) was 0.64 ± 0.023 1 kg-' after CC and 0.68 ± 0.0261 kg-1 after placebo treatment (P > 0.05). The zero order slope of the blood-ethanol decay profile (ko) decreased by about 5% when low Km ALDH was inhibited. The elimination of ethanol from the body (V x ko) was 1.9 ± 0.051 mmol kg-' h-1 after CC compared with 2.11 ± 0.056 mmol kg-' h-1 in placebo control experiments (P < 0.001). The area under the ethanol concentration time curve (0 -* 180 min) increased after CC treatment implying a change in clearance. 4 The disposition of acetaldehyde was markedly different in subjects pretreated with CC. The peak blood-concentrations, estimated by analysis of breath, ranged from pxmol I-1 compared with 1.7-6.5 ,umol F-1 after placebo. The apparent elimination half-life of acetaldehyde after inhibition of ALDH was 23 min on average with a range of 18-31 min. The area under the acetaldehyde-time curve (0 -> 180 min) was increased significantly after CC pretreatment. 5 The calcium carbimide-ethanol interaction caused intense facial flushing in all subjects tested beginning 20-30 min after drinking. Despite abnormally high concentrations of acetaldehyde in blood and breath after pretreatment with CC, the elimination kinetics of ethanol were not markedly changed from the placebo control trial. Our results do not support a significant role of acetaldehyde in regulating in vivo oxidation of ethanol in humans.Keywords ethanol acetaldehyde ALDH metabolism calcium carbimide kinetics breath analysis flush reaction