The first liquid chromatography-tandem mass spectrometric assay for the simultaneous determination of diclofenac, 4'-hydroxy-diclofenac, 5-hydroxy-diclofenac and diclofenac-acyl-glucuronide in mouse plasma, using a simple sample pre-treatment procedure, was developed and validated. Analytes in plasma were stabilized using acetic acid and ascorbic acid. After addition of the internal standard D(4)-diclofenac to a 10 microl sample volume and protein precipitation with acetonitrile, the supernatant was supplemented with an equal volume of water and injected into the chromatographic system. A polar embedded reversed-phase column with gradient elution using formic acid and ammonium acetate in water-methanol were used. The eluate was totally transferred into an electrospray interface with positive ionization and the analytes were quantified using triple quadrupole mass spectrometry. The assay was validated in the ranges 10-5000 ng/ml for 4'-hydroxy-diclofenac and 20-10,000 ng/ml for the other analytes, the lowest levels of these ranges (10 or 20 ng/ml) being the lower limits of quantification. Within day precisions were < or = 10%, between day precisions < or = 13% and accuracies were between 90 and 108%. The analytes were chemically stable under all relevant conditions. The assay was successfully applied in a pharmacokinetic study with diclofenac in mice.