Barbituric acid reacts with chloral hydrate in alkaline media, after heating for 1 min. at 100°C to give an orangish yellow colour having maximum absorbance at 450 nm. The reaction is selective for barbituric acid with 0.01 mg/ml as the visual limit of quantitation and provides a basis for a new spectrophotometric determination. The colour reaction obeys Beer's Law from 0.001 to 0.6 mg/ml and the relative standard deviation is 1.1%. The quantitative assessment of tolerable amounts of other drugs is also studied.