We examined differences between arterial and venous concentrations of nicotine in human subjects. Shortly after smoking a cigarette, levels of nicotine in arterial plasma were more than double those in venous plasma. The time course of the rise in arterial nicotine levels and the magnitude of the arteriovenous difference varied considerably among subjects. For some subjects, arterial nicotine concentrations after one cigarette were similar to venous concentrations typically observed after 20 cigarettes and were nearly 10 times greater than venous concentrations. Our findings have implications for understanding the high degree of addictiveness and cardiovascular toxicity of smoked forms of drugs.