Pentylenetetrazol administered to rats was metabolized to a derivative which was excreted in the urine along with unchanged pentylenetetrazol. Pentylenetetrazol and its metabolite can be separated on paper chromatograms developed in water-saturated isobutanol. The substances, however, are chromatographically inseparable with water as the mobile phase. Distribution of tritium-labelled pentylenetetrazol in the rat after intraperitoneal injection indicated that it was readily taken up by the liver. Perfusion of the isolated rat liver with citrated blood containing tritiated pentylenetetrazol demonstrated that the metabolite was formed in the intact liver. Formation of the metabolite was inhibited by SKF 525A. The metabolite was isolated from the urine of rats treated with pentylenetetrazol on an Amberlite XAD-2 column and elemental analysis showed it to be a sulfur-containing derivative.