Twelve pairs of MZ and 7 of DZ normal adult male twins were given a challenge dose of 0.8 mg ethanol per kg body weight diluted to a 30% v/v solution. Measures of blood alcohol, mood, craving for alcohol, body sway, heart rate and four psychomotor tasks were taken before, during and after intoxication. Genetic factors were found to be involved in the response of heart rate, body sway and two of the psychomotor tasks, but not in changes in blood alcohol, alertness or craving for alcohol. Drinking habits did not exert a strong influence upon acute responses to alcohol, but were significantly related to craving for alcohol whilst intoxicated.