The pharmacokinetics of flavone acetic acid (FAA) were comparatively studied in Balb-c mice and in immunocompetent nude mice treated with i.v. doses of 100 and 300 mg/kg. In both strains of mice, FAA kinetics appear to be dose-dependent, since the AUC values increased disproportionally to the dose. FAA protein binding and the pattern of distribution were similar in Balb-c and nude mice, the highest drug concentrations being found in liver and small intestine and the lowest in brain. Renal excretion appears to be the major route of FAA elimination in both strains, accounting for about 75% of the total drug administered after a dose of 100 mg/kg and 50-60% after a dose of 300 mg/kg. We conclude that FAA pharmacokinetics are comparable in Balb-c and nude mice so that the previously investigated FAA dosage schedules in inbred mice can also be employed in nude mice bearing human tumors.