We have analyzed the interrelationships between habitual alcohol consumption, peak blood alcohol concentration after a standard dose, and rate of alcohol metabolism in a group of 199 male and 213 female twins. Both peak concentration and rate of metabolism are strongly associated with alcohol consumption levels, even in the range of 0-10 g of alcohol/day. The peak concentration and rate of metabolism were strongly correlated in both men and women; this is not due to their common dependence on alcohol intake nor to experimental error. These results show that the threshold for effects of habitual consumption on alcohol pharmacokinetics is much lower than previously suspected, and that there are factors that reduce preabsorptive or first-pass metabolism but increase postabsorptive metabolism.