Nitric oxide (NO), a free radical gas implicated in a wide variety of biological reactions, is a novel signaling molecule that may regulate vasodilation, cerebral blood flow, and vascular permeability. This study was performed to determine whether NO mediates the selective increase in brain tumor microvessel permeability after intracarotid infusion of bradykinin in the RG2 rat glioma model. Intracarotid infusion of bradykinin selectively increased the transport of radiolabeled α-aminoisobutyric acid and dextran into brain tumors. Transport into normal brain was not increased. The administration of an NO synthase inhibitor, NG-nitro-l-arginine methyl ester, significantly inhibited the increased transport into tumors for both tracers. The inhibitory effect of NG-nitro-l-arginine methyl ester on the response to bradykinin was reversed by l-arginine. The expression of two NO synthase (NOS) isoforms in cultured RG2 glioma cell lines and intracerebral RG2 glioma was examined by immunohistochemistry and Western blot analysis. High levels of expression of neuronal NOS were detected in cultured and intracerebral RG2 cells but not in normal brain tissue, except in rare neuronal cells. The endothelial form of NOS was also expressed in cultured RG2 cells, but not as strongly as neuronal NOS expression. In intracerebral RG2 gliomas, expression of endothelial NOS in the tumor was detected at higher levels than in normal brain. These findings indicate that RG2 rat gliomas express high levels of NOS, which regulate the production of NO, compared with normal brain. We suggest that the selective permeability increase in brain tumor microvessels after bradykinin infusion is mediated by NO. Furthermore, the absence of high levels of NOS in normal brain may account for the attenuated permeability response to bradykinin in normal brain microvessels.