Cigarette smoke was introduced into the respiratory tract of artificially ventilated, anaesthetised dogs by a machine simulating the pattern of human smoking. A sensitive, specific GLC method was used to assay nicotine in smoke and in blood. The mean amount of nicotine in a puff of smoke was 120 μg; about 50% was abstracted in the respiratory tract. Nicotine was detected transiently in arterial blood after administration of single puffs of smoke; the peak concentrations were 0.15-0.25 μg/ml. After intravenous injection of 28 μg/kg of nicotine, it was detectable in arterial blood for 90-120 sec and peak concentrations were 0.9-1.3 μg/ml. The distribution between plasma and cells was in the ratio 1:0.8. The intensity and duration of the effects of smoke and injected nicotine on heart rate, blood pressure and tracheal pressure corresponded to the blood levels of nicotine. However, the depression of the knee jerk was more prolonged, suggesting localization of nicotine in the CNS. © 1969.