It has been known for many years that many physiological functions are proportional to the body weight when scaled across species. Similar allometric considerations generally hold for metabolic and renal clearances of compounds, and it can be expected for a given dose that small animals will generally have lower unchanged drug levels but higher metabolite levels than man. Acceptable safety margins between animals and man previously based on mg.kg.−1dosage may, therefore, need to be revised lower when using toxicokinetic measures of exposure. In the past, however, allometric scaling to man has required at least three species, and often the prediction of human clearance from this data has proved unreliable, minimizing its practical use in drug development. This preliminary review examines data from 51 drugs using various allometric methods and shows that an acceptable predictive accuracy, within 20%, may be found using the clearance corrected for differences in maximum life span obtained from Old World Monkeys. Further information on the suitability of a particular species can also be sought from comparisons of in vitro metabolic rates and profiles, and knowledge of specific cytochrome P450isozyme metabolism.