The stability of the antithyroid drug propylthiouracil was studied in an extemporaneously formulated suspension for 91 days. A simple, rapid, reproducible and stability-indicating reverse-phase high-performance liquid chromatographic assay method was developed for detecting propylthiouracil. The mobile phase consisted of a 70:30 (v/v) combination of water and methanol at a flow rate of 0.5 mL/minute. The detector wavelength was set at 276 nm. Excellent linearity (R squared value 0.9929) was obtained over the working solution range (1 to 100 micrograms/mL). The intraday and interday precisions of the assay method were o.11% to 1.28% and 1.65%, respectively. The drug was subjected to forced degradation under acidic and basic conditions, and the method proved to be stabililty-indicating for propylthiouracil. A suspension was formulated using commercially available 50-mg propylthiouracil tablets. The formulation consisted of a combination of 0.75% carboxymethylcellulose sodium and 0.5% Veegum as the suspending agents; 25% (w/v) simple syrup, 25% (w/v)sorbitol solution and 0.15% saccharin as the sweeteners; paraben concentrate as the preservative and a mixture of wild cherry, raspberry and tangerine as the flavor. The suspension was stored at 4, 30, 50, 60 and 70 deg C and analyzed on a weekly basis for 91 days. The samples were assayed in triplicate by the high-performance liquid chromatographic assay method outlined. The apparent zero-order rate constant for degradation of propylthiouracil at each of the temperatures was obtained and used to obtain the Arrhenius plot. From the regression line of the Arrhenius plot, the rate constant for degradation (t0.9) at room temperature (25 deg C) and under refrigeration (4 deg C) was found to be 0.078% day-1 and 0.04% day-1. The shelf life of the suspension was calculated to be 127 days at room temperature and 248 days under refrigeration.