A sensitive and simplified high performance liquid chromatographic procedure has been developed for the quantification of aceclofenac and its major metabolite in dog plasma. The recovery of aceclofenac was 91.3%-96.9%. The relative standard deviation was 3.69%-8.13%. The calibration curve was linear in the range from 0.050 microgram.ml-1 to 51.2 micrograms.ml-1 with gamma = 0.9998. The detection limit of the method is 10 ng.ml-1 for aceclofenac. Plasma concentrations of both aceclofenac and its major metabolite were simultaneously determined. After oral medication of aceclofenac to dogs at the dose of 2.5 mg.kg-1, the drug was absorbed rapidly and reached its peak level in plasma at about 12 min. The plasma drug concentration-time course conformed to a 2-compartment open model with a first order absorption. The mean value of T1/2 alpha and T1/2 beta for aceclofenac were 2.5 min and 137 min respectively. Plasma diclofenac concentration, a major metabolite of aceclofenac, reached its maximum at about 110 min, the mean Cmax for diclofenac was 3.2 micrograms.ml-1. The mean terminal elimination half-life for diclofenac was 140 min.