This study was conducted in male Sprague Dawley rats, maintained chronically on drinking water and on nicotine water at nicotine concentrations of 0.31 and 1.23 mM, to determine the effect of nicotine on body weight regulation and circulating growth hormone levels. The study was repeated to determine the effect of nicotine withdrawal on the same parameters after exposing the animals to high dose of nicotine (1.23 mM) for 12 weeks and then removing the nicotine from water for four weeks. Growth hormone (GH) and insulin levels in the plasma were measured by sensitive radioimmunoassays. The results show that with nicotine treatment the body weight, food and fluid intakes in rats decreased significantly, concomitant with the decrease in plasma glucose and insulin levels while plasma growth hormone levels were increased significantly. A pair-fed study conducted to determine the effect of decreased food intake showed no alteration of metabolic or hormonal parameters. Withdrawal of nicotine for four weeks reversed all of the previously noted parameters including plasma growth hormone levels; however, the body weights were not completely reversed. The data suggest that intake of nicotine affects body weights which appear to be associated with decreased plasma levels of glucose and insulin and release of growth hormone may play a role in that mechanism.