Human plasma levels of nicotine and its principal metabolite, cotinine, were simultaneously quantitated by gas-liquid chromatography combined with nitrogen selective detection. Nicotine, cotinine, and the added internal standard ketamine are extracted from plasma at basic pH into methylene chloride, back-extracted into acid, and then re-extracted into methylene chloride. Analysis is carried out on a packed glass column of 3% SE-30 while column temperature is programmed from 150 to 200 degrees C. Detector response is linea over the range of 2 to 50 ng/mL nicotine and 50 to 500 ng/mL cotinine. The method was validated on 150 plasma samples obtained from habitual smokers. Mean levels of 19.5 and 219 ng/mL were found for nicotine and cotinine, respectively. Both the mean and the range of the levels were in agreement with previously reported plasma levels for nicotine and cotinine.