A physiologically based pharmacokinetic (PBPK) model was developed to describe the disposition of nicotine in the Sprague-Dawley (SD) rat. Parameters for the model were either obtained from the literature (blood flows, organ volumes) or determined experimentally (partition coefficients). Nicotine metabolism was defined in the liver compartment by the first-order rate constants KNC and KNP which control the rate of nicotine metabolism to cotinine and "polar metabolites" (PM), respectively. These rate constants were estimated by optimizing the model fit to pharmacokinetic data obtained by administering an intraarterial (S)-[5-3H]nicotine bolus of 0.1 mg/kg to 6 rats. Model simulations that optimized for the appearance of cotinine in plasma estimated KNC and KNP to be 75.8 and 24.3 hr-1, respectively. Use of these constants in the model allowed us to accurately predict nicotine plasma kinetics and the fraction of the dose eliminated by renal (8.5%) and metabolic (91.5%) clearance. To validate the model's ability to predict tissue kinetics of nicotine, 21 male SD rats were administered 0.1 mg/kg (S)-[5-3H]nicotine intraarterially. At seven time points following treatment, 3 rats were euthanized and tissues were removed and analyzed for nicotine. Model-predicted nicotine tissue kinetics were in agreement with those determined experimentally in muscle, liver, skin, fat, and kidney. The brain, heart, and lung exhibited nonlinear nicotine elimination, suggesting that saturable nicotinic binding sites may be important in nicotine disposition in these organs. Inclusion of saturable receptor binding expressions in the mathematical description of these compartments resulted in better agreement with the experimental data. The Bmax and KD estimated by model simulations for these tissues were brain, 0.009 and 0.12; lung, 0.039 and 2.0; and heart, 0.039 nmol/tissue and 0.12 nM, respectively. This PBPK model can successfully describe the tissue and plasma kinetics of nicotine in the SD rat and will be a useful tool for pharmacologic studies in humans and experimental animals that require insight into the plasma or tissue concentration-effect relationship.