The stability of perphenazine in oral liquid dosage forms was studied by means of a stability-indicating high-performance liquid chromatography assay method that was developed in our laboratory. There was a direct relationship between the peak heights and the concentrations, with an r value of 0.998. The percent relative standard deviation based on five injections was 0.6. The products of decomposition and excipients present in the dosage forms did not interfere with the developed assay method. The dosage forms were stable for only 30 days when stored in amber-colored glass bottles at room temperature. The pH value of the mixtures remained constant at 3.7 after 90 days of storage, and the physical appearance of the mixtures did not change. The beyond-use date of 30 days is recommended for this procduct.