The aim of this study was to evaluate the effects of chronic nicotine administration on bone mineral homeostasis in rapidly growing young rats in comparison to effects in adult male rats. Two doses of nicotine (3 and 4.5mg/kg/day, as nicotine hydrogen tartrate) were used and rat treatment was continued for 6 months. In this study, all nicotine-treated rats weighed less than control rats and the effect was dose-dependent. Also, rats treated with nicotine had lower femoral wet weight and showed a significant reduction in femoral mid-shaft cortical width and femoral and lumbar vertebral ash weights. These effects were associated with a significant reduction of ash calcium and phosphorus contents of the femora and lumbar vertebrae. The bone mineral-lowering effects of nicotine were more severe in the lumbar vertebral spongy bone than in the femoral compact bone and these changes were more marked in adult rats than in young rats. An additional interesting observation was that the femora of young rats treated with nicotine were significantly shorter than those of control young rats. Also, the values of the femoral ash weight per unit length were significantly decreased in nicotine-treated adult rats but not in nicotine-treated young rats. Thus, these results show that nicotine-induced changes in bone vary with age. The clinical relevance of this study is that it may provide justification to insist that all people in general and the risky young group in particular should be warned against the hazards of the negative effects of nicotine on bone.