The dose-dependent pharmacokinetics of caeic acid (CA) were studied in rabbits. Three dierent doses (5, 10, and 25 mg kg 71 ) were administered intravenously to six rabbits each. The concentrationยฑtime proยฎles for CA could be ยฎtted by a twocompartment model for each dose. The results showed that total-body clearance and elimination rate constant from the central compartment (k 10 ) after a 5 mg kg 71 dose were greater than those after the other two doses. Furthermore, the terminal elimination half-life (b half-life) and mean residence time (MRT) after a 5 mg kg 71 dose were less than after the other doses. The AUC value increased linearly with dose within the range of 10ยฑ25 mg kg 71 . Most of the unchanged caeic acid was excreted in the urine within 2 h. The percentage of unchanged caeic acid excreted in the urine was 63ยด4, 60ยด0, and 55ยด4% after doses of 5, 10, and 25 mg kg 71 , respectively, which was not signiยฎcantly dierent. However, signiยฎcant dierences in the renal clearances and renal excretion rate constants were observed with a 5 mg kg 71 dose compared to the other doses. On the other hand, nonrenal clearances and nonrenal excretion rate constants showed no doserelated dierences. The dierences observed in total-body clearance, k 10 , b half-life, and MRT between a 5 mg kg 71 dose and the other doses can be explained on the basis of the dierences in renal clearance and renal excretion rate constants. &1997 John Wiley & Sons, Ltd.