A sensitive and specific method for the determination of 5-(2,5-dimethylphenoxy)-2,2-dimethyl-pentanoic acid (gemfibrozil, Gevilon) at therapeutic concentrations in plasma is described. The method is based on high performance liquid chromatography and the use of ibuprofen as internal standard. Gemfibrozil and the internal standard are extracted from acidified plasma into cyclohexane and the resulting residue is analyzed on a commercial reversed phase column with acetonitrile/water 1:1 and 0.2% phosphoric acid as mobile phase. The eluted peaks are detected by UV-absorption at 225 nm. The sensitivity is approx. 50 ng/ml plasma using a 0.5-ml sample. The applicability to pharmacokinetic studies of gemfibrozil is demonstrated.