BACKGROUND: This clinical study was conducted to compare pharmacokinetics of eperisone and aceclofenac of HCP1104, a new fixed dose combination drug with those in co-administration of eperisone 50 mg and aceclofenac 100 mg. The study used a partial replicated study design to characterize intra-subject variability of eperisone when co-administrated with aceclofenac. METHODS: A partial replicated crossover design was employed in 30 subjects. Each subject received a single dose of co-administration of eperisone 50 mg and aceclofenac 100 mg on two occasions and a single dose of 1 capsule of HCP1104. Blood samples were obtained for 24 hrs after dosing, and plasma was assayed for eperisone and aceclofenac by Liquid chromatography-electrospray ionization-mass spectrometry. RESULTS: Using an average bioequivalence criterion, the 90 % confidence limits for Ln-transformed Cmax and AUClast for aceclofenac fell wihin the acceptable range of 80 - 125 %. Point estimates of eperisone AUClast and Cmax were 1.0152 and 1.0490, respectively and the 90 % confidence interval for Cmax was 0.8499 - 1.3025. The within-subject coefficient of variation of Cmax for the reference was 50.198 %. Acceptance range for eperisone Cmax based on new bioequivalence guidance for highly variable drugs was extended to 0.6984 - 1.4319. CONCLUSION: The extent of exposure and rate of absorption of both eperisone and aceclofenac with a single dose of HCP1104 capsule were equivalent to those with co-administration of a marketed eperisone 50 mg tablet and a marketed aceclofenac 100 mg tablet under fasting conditions in healthy adult males.