(1) To determine the disposition kinetics of nicotine and cotinine, including the fractional conversion of nicotine to cotinine, (2) to compare the disposition kinetics of deuterium-labeled and unlabeled cotinine, and (3) to develop a pharmacokinetically based method for estimating daily intake of nicotine from cigarette smoking. Twenty cigarette smokers received a combined infusion of deuterium-labeled nicotine (d2) and cotinine (d4). Six nonsmokers received a combined infusion of unlabeled cotinine, cotinine-d2 and cotinine-d4. Daily intake of nicotine was estimated with use of the plasma cotinine concentration during ad libitum smoking, clearance of labeled cotinine, and fractional conversion of nicotine to cotinine. The kinetics of labeled versus unlabeled cotinine and of cotinine in smokers versus nonsmokers were similar. On average, 72% of nicotine was converted to cotinine, with a range from 55% to 92%. Subjects with lower clearances of nicotine had lower fractional conversion of nicotine to cotinine, indicating that this is the most rapid of the proximate metabolic pathways for nicotine. The equation for estimating daily intake of nicotine from smoking was: Dnic (mg/24 hr) = K x (Plasma Cot) (ng/ml), where K averaged 0.08, with a range from 0.047 to 0.102. Individual variability in the clearance of cotinine (coefficient of variation, 27.5%) accounts for more of the variability in K than does variability in the fractional conversion of nicotine to cotinine (coefficient of variation, 12.3%). Our study provides quantitative data on individual variability in the extent of C-oxidation of nicotine to cotinine and a quantitative perspective on the use of plasma cotinine as an indicator of daily intake of nicotine from tobacco.