The effect of transdermal nicotine patches on ad libitum cigarette smoking was examined in 30 subjects by measuring behavioural, biochemical and subjective aspects of smoking during a week of smoking without patches, and then a week each of nicotine and placebo patches in a randomised double blind crossover design. While wearing nicotine patches the subjects did not reduce the number of cigarettes smoked, but their expired carbon monoxide was reduced by 14%, they obtained less satisfaction from their cigarettes, and reported fewer and weaker urges to smoke. Down-regulation of nicotine intake from cigarettes was imprecise, such that when subjects wore nicotine patches their post-cigarette plasma nicotine concentration increased to an average of 45 ng/ml compared with 37 ng/ml in both no patch and placebo patch conditions. As the nicotine patches produced a plasma nicotine concentration of 15.9 ng/ml in abstinent subjects, this suggests a 22% reduction in nicotine intake from cigarettes while wearing nicotine patches. No serious symptoms of nicotine overdose were reported. It is suggested that the continuous absorption of nicotine from the patch may cause a build-up of acute tolerance to both toxic and pleasant subjective effects from smoking.