The author describes a method of using reversed-phase high-performance liquid chromatography with fluorimetric detection for the assay of diclofenac sodium in serum. The method is sensitive down to 20 ng/ml (250-microliters loop). Elution is at pH 6.2 with methanol in 0.05 M phosphate buffer (43:57, v/v) on a 25-cm Spherisorb S5 ODS2 column. Detection is at an excitation wavelength of 282 nm and an emission wavelength of 365 nm. Serum sample size is 100 microliters. Sample protein, to which diclofenac is highly bound, is first denatured by heat and then with methanol to release the diclofenac prior to centrifugation and injection of 100 microliters (or 250 microliters) of the clear supernatant. Harmol, with similar fluorescence and polarity characteristics to diclofenac, is a satisfactory internal standard. At the 1 micrograms/ml level intra-sample reproducibility is better than 2%, whilst inter-sample reproducibility is 4.6%. Detector response is linear from 40 ng/ml to 20 micrograms/ml (100-microliters loop).