Daily intake of nicotine in 22 subjects was estimated from metabolic clearance data obtained after intravenous infusion of nicotine and from blood and urinary nicotine concentration data obtained over 24 hr when the subjects were smoking cigarettes. Daily intake of nicotine averaged 37.6 mg (+/- 17.7, SD) but varied widely among subjects (10.5 to 78.6 mg). Men metabolized nicotine faster than did women, but daily intake of nicotine did not differ. Intake correlated strongly with cigarettes smoked per day (r = 0.59) but not with machine-determined yield. Nicotine intake per cigarette averaged 1.04 mg (+/- 0.36) but did not correlate with machine-determined yield. Correlations between several commonly used biochemical markers of tobacco smoke and nicotine intake were examined; the afternoon (4:00 P.M.) blood level of nicotine was the best marker.