The effect of cigarette smoking on drug oxidation and conjugation was studied using antipyrine and acetaminophen as marker compounds. For the antipyrine study, healthy cigarette smokers (n = 30) and nonsmoking controls (n = 53) received a single 1.0-gram intravenous dose of antipyrine. For the acetaminophen study, 14 smokers and 15 nonsmokers received a 650-mg intravenous dose of acetaminophen. The clearance of antipyrine was significantly increased (0.93 vs. 0.60 ml/min/kg, p less than 0.0001) and elimination half-life was correspondingly reduced (8.9 vs. 13.0 h, p less than 0.0001) in smokers compared to nonsmoking controls. Total recovery of antipyrine and metabolites excreted in urine did not differ between groups, but there was a significantly increased fractional clearance of antipyrine via formation of 4-hydroxyantipyrine and 3-hydroxymethyl metabolites in smokers. Fractional clearance via formation of norantipyrine did not differ significantly between groups. Comparison of acetaminophen kinetics between smokers and nonsmokers indicated no significant differences in elimination half-life, clearance or volume of distribution. Thus, cigarette smoking is more likely to induce drug oxidation rather than drug conjugation. However, not all oxidative pathways are equally influenced; induction effects of smoking are highly substrate selective and pathway specific.