The metabolism and excretion of imrecoxib, a novel and moderately selective cyclooxygenase-II inhibitor, were investigated in rat. The structures of metabolites were identified by mass spectrometry (MSn) and nuclear magnetic resonance. Metabolic profiles of imrecoxib in urine, bile and faeces were obtained by HPLC and LC/MSn, and cumulative excretion was determined by LC/MSn. Imrecoxib was extensively metabolized in rat after intravenous administration, with less than 2% of the dose excreted as parent drug in either urine or faeces. The major metabolic pathway was that the 4'-methyl group of imrecoxib was first oxidized to the 4'-hydroxymethyl metabolite (M4), followed by additional oxidation to 4'-carboxylic acid metabolite (M2). The dihydroxylated metabolite, 4'-hydroxymethyl-5-hydroxyl imrecoxib (M3), was further oxidized to 4'-hydroxymethyl-5-carbonyl metabolite (M5), and glucuronide conjugates of M2-4 were formed. After intravenous (5 mg kg-1) administration, the majority of the dose was recovered in the faeces. The dose was primarily excreted as the carboxylic acid metabolite in addition to the 4'-hydroxymethyl metabolite. The carboxylic acid metabolite was mainly excreted in faeces, while the 4'-hydroxymethyl metabolite was mainly excreted in urine.