Six volunteer female habitual smokers were exposed during a 2-wk experimental period to cigarette smoke, both actively and passively, in an exposure chamber (volume 10 m3, average air exchange rate 6.8 times/h), where the ambient carbon monoxide, particle, and aldehyde concentrations were monitored. Three of the six subjects were smoking at the time, 2 cigarettes (filtered, self-burning low tar brand) per person per hour, 30 cigarettes altogether during each of the 5-h experimental days in the chamber. Samples of blood and urine were taken from each subject after 3 nonsmoking days and after each day of active or passive smoking. Among the parameters tested, blood carboxyhemoglobin, plasma cotinine, and urinary mutagenicity were higher in samples taken after active smoking than after nonsmoking periods. Although the exposure conditions were similar for all subjects, the parameters measured showed quite high interindividual variation. Thioethers and thiocyanates were not significantly elevated in the active smoking samples; neither were there any differences during this short experimental period in the sister chromatid exchange frequencies. The only parameters showing an increasing trend after passive exposure, as compared with nonsmoking samples, were urinary mutagenicity and plasma cotinine, the main metabolite of nicotine.