A rapid and sensitive capillary gas-chromatographic method with nitrogen-sensitive detection is reported for the simultaneous analysis of nicotine and cotinine levels occurring in the plasma, saliva, and urine of regular tobacco smokers. The proposed assay has a linear output, has satisfactory accuracy over the range of concentrations of both amines encountered in active smokers, and has also been successful in the analysis of the urine samples of passive smokers. Its lower limit of sensitivity is 0.2 ng of nicotine and 0.5 ng of cotinine per ml of plasma or saliva or per 100 microliters of urine. The beneficial characteristics of the presented method were achieved by the combination of solid phase extraction of 0.1-1.0 ml of fluid specimens, capillary column gas chromatography with splitless injection and nitrogen sensitive detection, and the use of separate, structurally analogous compounds as internal standards for nicotine. The suitability of the assay is shown by plasma concentration-time curves of nicotine and cotinine in a steady smoker during a 24 hours period.