In six male alcoholic volunteers, oral administration of calcium carbimide (0.7 mg/kg) before ingestion of ethanol (0.5 g/kg) produced an interaction consisting of increased blood acetaldehyde level, tachycardia and increased pulse pressure, which was due mainly to decreased diastolic blood pressure. For these experimental conditions, calcium carbimide had a duration of action of at least 24 h to produce an interaction with ethanol. The order of intensity of the interaction with regard to the calcium carbimide pretreatment time interval was 4 greater than 8 approximately 12 greater than 24 h. Using the criterion of heart rate above 100 as indicative of the calcium carbimide-ethanol interaction, the onset was 0.13, 0.25, 0.25 and 0.38 h for the 4-, 8-, 12- and 24-h pretreatment experiments and the duration of the interaction was 1.6, 1.0, 1.0 and 0.12 h, respectively. There were positive linear correlations between acetaldehyde level and heart rate and between acetaldehyde level and pulse pressure. There was appreciable interindividual variability in the heart rate and blood pressure responses. Increased blood acetaldehyde level seemed to be required for the physiological changes to occur. Calcium carbimide pretreatment at the 4-h interval produced increased blood ethanol level for the last hour of the interaction and reduced the rate of ethanol metabolism.