Our results concern the regional localization of rhodanese in the mouse brain. A histoenzymatic examination was undertaken in telencephalon, diencephalon, mesencephalon, and rhombencephalon. The sites of rhodanese activity are revealed as punctuate, granular, dark dots, small in some regions such as hippocampus or bigger in others, and as long, thread-like particles especially, abundant in the region of the telencephalon in the astroglia cells and in the region of the mesencephalon in the hippocampus. There were sites with a high density of the histochemical test products, for example, the ependymoma of the forth cerebral ventricle, choroid plexus, and nerve ducts. These findings support the detoxifying role of rhodanese in brain regions.