A sensitive, specific and precise high-pressure liquid-chromatographic method for the determination of the analgesic agent, zomepirac, in plasma samples is described. The lowest concentration of zomepirac that can be measured accurately and precisely (coefficient of variation less than 20%) in a 2 ml plasma sample is 10 ng/ml. The standard curve is linear in the concentration range of 10 to 5000 ng/ml. To date, this procedure has been employed successfully in analysing over 10,000 clinical plasma samples.