Nicotine-exposed and control rats were compared with respect to contractile, enzyme-histochemical and biochemical properties of fast- and slow-twitch skeletal muscles in order to elucidate the mechanisms underlying previously observed effects of tobacco smoking on skeletal muscle. The nicotine was administered in drinking water, since this approach has been shown to result in a plasma nicotine pattern similar to that seen in tobacco smokers. In a pilot study, fibre-type proportions and mitochondrial enzyme activities tended to change in the slow-twitch soleus muscle after 9 weeks of nicotine exposure in a way similar to that previously reported in tobacco smokers. In the present study, the duration of nicotine exposure was more prolonged (18 weeks) and the number of studied animals was increased. In this series neither contractile, enzyme-histochemical nor biochemical properties were affected by the nicotine exposure. It is thus concluded that prolonged nicotine exposure has no significant effect on the skeletal muscle characteristics studied, and that other aetiological agent(s) for the observed differences in such characteristics between smokers and non-smokers should be searched for.