The separate effects of tar and nicotine on the cigarette smoking manoeuvre were investigated. Each of ten asymptomatic habitual smokers smoked three different commercially available cigarettes in a randomised order. The brands were chosen such that two had the same tar yield (10 mg) and two had the same nicotine yield (1.4 mg). The volume of smoke inhaled into the lungs was measured by tracing the smoke with the inert gas 81Krm. Puffing indices were recorded using an electronic smoking analyser and flowhead/cigarette holder. There was no difference in the total volume of smoke puffed from each of the cigarette brands. With cigarettes of the same tar level, the total inhaled smoke volume was lower with the higher nicotine cigarette (P less than 0.05): by contrast, with cigarettes of the same nicotine level, the total inhaled smoke volume was lower with the lower tar cigarette (P less than 0.02). Tar and nicotine appear to exercise independent control over the volume of smoke inhaled.