Nitric oxide (NO), which is synthesised by the enzyme Nitric Oxide Synthase (NOS), mediates many physiological and pathological mechanisms in the brain. Experimental studies of rodent C6 glioma show that NO has a major role in regulation of tumour blood flow. To determine the relevance of these findings to human malignant glioma, NADPH diaphorase (NADPHd) histochemistry, which is a marker for NOS expression, was performed in 20 glioblastomas. Except for one tumour which was totally necrotic, all the 19 tumour specimens showed evidence of NADPHd expression. The neoplastic vascular endothelium, areas of endothelial proliferation and neoplastic astrocytes all consistently showed high levels of NADPHd positivity. Areas of necrotic tumour were always NADPHd negative. Both the extent and the intensity of cellular staining within the glioblastoma was considerably greater than NADPHd positivity in normal brain tissue. These results together with findings in experimental glioma strongly suggest that NOS has a definite role in the pathophysiology of glioblastoma and that it may be possible to pharmacologically manipulate them for therapeutic benefit.