Aceclofenac is an orally effective non-steroidal anti-inflammatory agent of the phenylacetic acid derivative. Bioequivalence study of two aceclofenac preparations, the test drug (Senafen^R: Daewon Pharmaceutical Company) and the reference drug (Airtal^R:Daewoong Pharmaceutical Company), was conducted according to the guidelines of Korea Food and Drug Administration (KFDA). Sixteen healthy male volunteers, 24 ±4 years old and 63.9 ±6.9 kg of body weight in average, were divided randomly into two groups and administered the drug orally at the dose of 100 mg as aceclofenac in a 2 x 2 crossover study. Plasma concentrations of aceclofenac were monitored by HPLC method for 12 hr after administration. AUC_(0-12h) (area under the plasma concentration-time curve from initial to 12 hr) was calculated by the linear trapezoidal method. C_(max) (maximum plasma drug concentration) and T_(max) (time to reach C_(max)) were compiled directly from the plasma drug concentration-time data. Student`s t-test indicated no significant differences between the formulations in these parameters. Analysis of variance (ANOVA) revealed that there are no differences in AUC_(0-12h), C_(max) and T_(max) between the formulations. The apparent differences between the formulations were far less than 20% (e.g., 0.25, 0.01 and 7.32 for AUC_(0-12h), C_(max) and T_(max), respectively). Minimum detectable differences (%) between the formulations at α=0.05 and 1-β=0.8 were less than 20% (e.g., 14.65, 12.47 and 15.46 for AUC_(0-12h) C_(max) and T_(max) respectively). The 90% confidence intervals for these parameters were also within ±20% (e.g., -10.19∼10.68, -8.87∼8.89 and -3.69∼18.33 for AUC_(0-12h), C_(max) and T_(max), respectively). These results satisfy the bioequivalence criteria of KFDA guidelines, indicating that two formulations of aceclofenac are bioequivalent.