Only recently has attention been focused on the importance of interethnic differences as determinants of interindividual variability in drug response. We compared the pharmacokinetics and pharmacodynamics of morphine in eight Chinese and eight white healthy mean after 0.15 mg/kg of morphine intravenously. The clearance of morphine was significantly higher in the Chinese subjects than in the white subjects because of an increase in the partial metabolic clearance by glucuronidation. There was no interethnic difference in the metabolism to normorphine. Morphine depressed the respiratory response to rebreathing carbon dioxide more in white subjects than in Chinese subjects, resulting in a greater reduction in resting ventilation and resting end-tidal PCO2. The slope of the ventilation/PCO2 response curve, a measure of carbon dioxide sensitivity, was reduced more in white subjects than Chinese subjects. As a result, white subjects had a greater depression in ventilation at a PCO2 of 55 mm Hg. The morphine-induced reduction in blood pressure was also greater in white subjects than in Chinese subjects. Thus this study has shown ethnicity to be an important determinant of the disposition and effects of morphine.