The chemical stability of isoniazid in an oral liquid dosage form was studied by means of a specially developed stabilty-indicating high-performance liquid chromatographic assay method. The concentrations of the drug were directly related to peak heights, and the percent relative standard deviation based on five injections was 0.93. The products of decomposition and excipients present in the dosage forms did not interfere with the developed assay method. The preparation was stable for at least 42 days when stored in amber-colored glass bottles at room temperature. The pH value of the preparation decreased from 5.9 to 5.6 after 42 days of storage. The physical appearance of the preparation changed from almost colorless to light brown.