In order to determine the pathway of extracellular metabolism of the thyrotropin releasing hormone (pyroglu-his-proNH(2)) in brain, the topographical organization of pyroglutamate aminopeptidase II on the plasma membrane was investigated. Its activity was only slightly increased when intact brain synaptosomes were lysed by osmotic shock or detergent treatment. Trypsin treatment of intact synaptosomes destroyed 70-80% of enzyme activity without affecting lactate dehydrogenase. Pyroglutamate aminopeptidase II activity was present in primary cultures of foetal mice cortical cells. It was detected in intact cells, was not released by the cells and its activity was not increased by saponin pretreatment. Trypsin treatment of the cells reduced pyroglutamate aminopeptidase II by 70% but did not affect pyroglutamate aminopeptidase I and lactate dehydrogenase. These data support that brain pyroglutamate aminopeptidase II is an ectoenzyme. They suggest that this enzyme could be responsible for thyrotropin releasing hormone extracellular catabolism in brain.