Pentylenetetrazol is quantitatively precipitated from aqueous solution by cuprous chloride reagent. The pentylenetetrazol-cuprous chloride complex is dissolved in concentrated nitric acid; the solution is adjusted to pH 4 with acetate buffer and sodium hydroxide solution, and the deep blue color of the copper complex developed with tetraethylenepentamine is determined photometrically at 580 mμ The method can be applied to samples containing from 2–10 mg. pentylenetetrazol and furnishes a rapid means of determining small amounts of the drug in the presence of other ingredients in pharmaceutical preparations.