A gas chromatographic method has been established for the quantitative analysis of nalidixic acid. The method is based on the derivatization of nalidixic acid with diazomethane, and 5-alpha-cholestane is used as an internal standard. The sample is chromatographed on a glass column packed with 1% OV-1 on Chromosorb W. Quantitation is achieved by measuring peak-height ratios. The improved simplicity, specificity and accuracy of the method has been demonstrated for the quantitation of nalidixic acid in tablets.