The chemical stability of thiopental sodium injection (2.5%) when stored at 25° and 5° in disposable plastic syringes of two manufacturers (Monoject and Becton Dickenson and Co.) has been studied using the USP‐NF method. The injection appeared to be stable for five days at 25° and 45 days at 5° with a loss in potency of less than 7%. The thiopental sodium did not adsorb on the syringes. The pH values and the physical appearance did not change. An additional peak was obtained in the chromatogram from both the freshly prepared and the assay solution probably due to an impurity in the powder.