To elucidate the role of nitric oxide (NO) in synaptic structures, the ultrastructural localization of the enzyme NO synthase (NOS), based on the cytochemical NADPH-diaphorase staining, was studied in neocortical and hippocampal neuropil areas. For the localization of NADPH-diaphorase activity a special tetrazolium salt (BSPT) was applied. BSPT-formazan, the osmiophilic reaction product, was found to be attached to endomembranes, predominantly the endoplasmic reticulum. Quantitative studies on synaptic regions revealed that mainly presynaptic areas (41% in hippocampus, 38% in neocortex) showed labelling. Postsynaptic regions were only exceptionally labelled by BSPT-formazan (3% in hippocampus, 5% in neocortex). The present findings support the view that NO may be involved in diverse synaptic functions acting preferentially from the presynaptic side.