The purpose of the study was to examine long-term nicotine substitution and its variability during use of a nicotine patch. In two smoking cessation studies a 16-h nicotine patch, releasing 15 mg nicotine, was applied daily for 16 h over 12 weeks, to 167 smokers. Salivary cotinine was highly correlated with plasma cotinine (r = 0.93), and the concentration of cotinine in a single sample in the afternoon was well correlated with the AUCcontinine over 24 h (r = 0.94). The salivary cotinine concentration after 1 week in 60 abstainers was 183 ng.ml-1. After 3,6 and 12 weeks the cotinine concentrations were 86%, 79% and 59% of the 1-week value. The degree of nicotine compensation attained by the patch after 1 week was 52% (SD 24%) in subjects who succeeded in stopping smoking for at least 3 weeks. A quarter of the subjects achieved a compensation of less than 35% of their usual nicotine intake. Nicotine substitution with this 16-h nicotine patch was stable and the risk of overcompensation was small in this group of smokers.