A reversed‐phase high‐performance liquid chromatographic method with electrochemical detection for the quantitative determination of diclofenac potassium in plasma was developed. Naproxen was used as the internal standard. The drug and internal standard were isolated from plasma by extraction with dichloromethane and 2 M hydrochloric acid. Chromatographic separation was performed on a C₁₈ column with methanol_water (68:32, v/v) adjusted to pH 3.2 with phosphoric acid as mobile phase. The oxidation potential for detection was established by constructing a voltammogram for diclofenac. The quantification limit for diclofenac in plasma was 5 ng mL⁻¹. Linearity of the method was confirmed in the range 5_2000 ng mL⁻¹, correlation coefficient 0.9998. Within‐day relative standard deviations (RSDs) ranged from 0.66 to 14.00% and between‐day RSDs from 0.59 to 15.78%. The method was successfully applied for the determination of pharmacokinetic parameters after ingestion of a 50 mg dose of diclofenac. Studies were performed on 18 healthy volunteers of both sexes. Copyright © 2005 John Wiley & Sons, Ltd.