Electron-microscopic, histochemical and endocrinologic study of aldehyde-fuchsin-positive (Gomori-positive; GP) grains of rat brain periventricular glia (GP glia) was carried out. GP structures appear as a population of osmiophilic particles, which is heterogeneous in both shape and size. Laminar structures interspersed with fine granular material were seen in the GP granules. No activity of the lysosomal and mitochondrial enzymes could be observed. The reaction for peroxidase was also negative. The GP material was stained with PAS and Ziehl-Nielsen. There are apparently no lipoid inclusions in the GP grains. The primary red-orange fluorescence distinguishes the GP glia from other structures in the rat brain. So GP grains are a specific cytoplasmic formation having some similarity to lipofuscin. There was a considerable decrease in GP grains after administration of estradiol in ovariectomized rats and also in pregnant rats. Dopamine administration and starvation caused some reduction in GP grains. In the rat hypothalamus, distribution of the main mass of GP glia corresponds with the so-called hypophysiotropic area. The possible participation of GP glia in the neuroendocrinological process is discussed.