We reanalyzed data on the decline in performance on a battery of psychomotor tests, after a standard dose of ethanol (0.75 g/kg body weight), of 206 same-sex twin pairs. Principal components analysis identified two orthogonal factors. The first factor was strongly associated with increased body sway, self-rated intoxication and unwillingness to drive, and reported low average weekly alcohol consumption, but showed a very weak association with blood alcohol concentration. The second factor had high loadings on tests assessing psychomotor coordination, was strongly associated with blood alcohol concentration, but was unrelated to willingness to drive or self- rated intoxication. Multivariate genetic analysis indicated independent genetic and environmental determination of differences in sensitivity to the effects of alcohol on these two factors.