After intraventricular administration on 14C- or 3H-labelled taurine, gamma-glutamyl-taurine was detected in the protein free extract of rat brain using a combination of ion exchange chromatography, electrophoretic separation and thin-layer chromatography. Under in vitro conditions gamma-glutamyl-transpeptidase (EC 2.3.2.2.) prepared from rat brain catalyses the formation of gamma-glutamyl-taurine from taurine and glutathione or gamma-glutamyl-p-nitroanilide. The enzymes of the gamma-glutamyl cycle are presumed to be responsible for the in vivo metabolism of this dipeptide.