Propylthiouracil (2-thio-4-oxo-6-propy1-1,3-pyrimidine) reacts with sodium nitroprusside in presence of sodium hydroxide, and on treatment with acetic acid it gives a greenish blue color measurable at 600 nm. The effects of pH, time, concentrations of sodium nitroprusside, sodium hydroxide and acetic acid have been studied. The developed color is used as a basis of a colorimetric method for the determination of pure propylthiouracil and in pharmaceutical formulations. The method can be adopted for concentrations ranging from 8 to 32 μg per ml with 100% ± 2.0 accuracy.