A High Performance Liquid Chromatographic (HPLC) method was developed for the analysis of Bay 12-9566 (4-[4-4-(chlorophenyl)Phenyl]-4-oxo-2S-(phenylthiomethyl) butanoic acid), a metalloproteinase inhibitor, and its three metabolites, M1 (sulfoxide), M2 (ρ-hydroxy) and M3 (reduction product) in plasma. Sample preparation involved precipitation of plasma proteins using acidified acetonitrile. A reverse phase chromatography with gradient elution, ultraviolet detection at 290 nm and internal standard were used for separation and quantitation of all analytes. The quantitation range was 0.10 μg/mL to 50 μg/mL for BAY 12-9566, and 0.10 μg/mL to 3.00 μg/mL for all three metabolites. The limit of detection for BAY 12-9566 and for all three metabolites was 0.05 μg/mL. Intra-day accuracy for BAY 12-9566 and the three metabolites ranged from 95.0 to 105.0% and precision (CV) ranged from 0.0 to 4.76%.