In the present study we have used electron microscopical NADPH-diaphorase (NADPH-d) histochemistry as a visualization procedure for nitric oxide synthase (NOS) to examine patterns of activity in the subcellular distribution of NADPH-d in the rat striatal astroglia. Electron microscopical examination revealed deposition of the NADPH-d reaction product in a nuclear envelope, fragments of endoplasmic reticulum and mitochondria. Predominantly mitochondria of astrocytic "end feet" were labelled. Our ultrastructural observations promote the possibility that astroglial NADPH-d/NOS is involved in adaptation of the local blood flow in the striatal microenvironment.