The viewpoints and methodologies of pharmacokinetic modeling in toxicology and pharmaceutics are divergent. In order to appreciate both approaches, each method was used to model the kinetics of various compounds. The pharmacokinetics of the nucleoside analog (2S, 4R)-1-2-[2- (hydroxymethyl)-1,3-dioxolan-4-yl]-5-iodouracil (IOddU), a potent in vitro antagonist of Epstein-Barr virus replication, were investigated. Total (mean ( rsd)) and renal clearance after iv administration were 1.44 (0.588) and 0.69 (0.46) L/h/kg in six male Sprague-Dawley rats. Volume of distribution was 0.643 (0.619) L/kg. MRT (h), lz (min- 1), and fraction of dose excreted in urine were 0.510 (0.527), 1.08 (0.714), and 47.8%. Iohexol is a radio contrast agent eliminated solely by glomerular filtration. In order to support development of a clean, simple, and precise means of determining glomerular filtration rate in dogs, the plasma clearance of iohexol was compared with urinary creatinine clearance at different levels of obesity (lean, fattened, and obese). Absolute plasma clearance of iohexol increased with obesity as expected; body weight adjusted plasma clearance of iohexol did not increase significantly. A three point method sampling in the terminal phase overpredicted urinary creatinine clearance slightly. The nucleoside analogs, 3'-azido-3'-deoxythymidine and (-)-2',3'-dideoxy-3'- thiacytidine (AZT, 3TC) are potent inhibitors of feline immunodeficiency virus (FIV) for which lymph tissues serve as viral sanctuaries. The lymphatic disposition of AZT and 3TC in cats were determined. Lymph tissues included were tonsil, thymus, submandibular, bronchiolar, sternal, and mesenteric lymph nodes. Mean overall lymph tissue concentrations of AZT and 3TC were 8.13 (0.79) and 7.74 (0.66). Tissue to plasma concentration ratios were 0.36 (0.76) and 0.44 (0.51) for AZT and 3TC. Bromodichloromethane is a hepatic carcinogen present in chlorinated drinking water. A physiologically based pharmacokinetic model was developed to simulate concentrations in plasma and tissues of the Japanese Medaka fish (Orynchias latipes). Incidence of hepatocellular adenoma in male medaka exposed at three levels (0, 1.5, 15 mg/L) correlated very well (r2=1.00) with weekly averaged simulated area under the concentration versus time curve.