The cellular localization and distribution of inducible and constitutive nitric oxide synthase (iNOS/cNOS) was determined in tissue sections from multiple sclerosis (MS) and control brain and spinal cord. Immunocytochemical techniques were applied using specific iNOS- and cNOS-directed antibodies. In addition, NADPH-diaphorase histochemistry was performed. To establish the identity of iNOS-, cNOS- and NADPH-diaphorase-positive cells single and double staining was performed on tissue sections with the macrophage marker KP1 (CD68) and with the astrocyte marker glial fibrillary acidic protein (GFAP). Areas of myelin breakdown and demyelination were determined using a staining for neutral lipids, Oil Red O (ORO). Furthermore, macrophages isolated from active demyelinating MS lesions were stained for iNOS, cNOS, KP1 and ORO. In active MS lesions strong iNOS immunoreactivity was found exclusively in perivascular and parenchymal macrophages distributed within regions of active demyelination. In these active MS lesions immunoreactivity for cNOS was also found in macrophages. Macrophages isolated from active MS lesions also showed immunoreactivity for iNOS and cNOS. Moreover, these isolated macrophages produced nitric oxide (NO; >30 microM) in vitro. NADPH-diaphorase activity was detected in KP1-positive perivascular and parenchymal macrophages and in GFAP-positive reactive astrocytes in active MS lesions and in reactive astrocytes located in the hypercellular rims of chronic active MS lesions. cNOS-positive reactive astrocytes were detected in both active and chronic active MS lesions. Inside chronic active lesions some residual macrophages were weakly iNOS-positive. In control brain and spinal cord no iNOS immunoreactivity could be detected. These results suggests an important role for human macrophages capable of producing the free radical nitric oxide (NO), which may contribute to the cytotoxicity of oligodendrocytes and destruction of myelin in MS brain and spinal cord.