C6 glioma strongly express nitric oxide synthase. Rats bearing C6 tumours were pre-treated with i.v. Ng-nitro-L-arginine methyl ester (L-NAME), 3-morpholinosydnonimine (SIN-1) or saline before local cerebral blood flow (LCBF) or tumour capillary permeability (TCP) was measured by the [14C]iodoantipyrine autoradiographic or [14C]α-amino-isobutyric acid techniques. L-NAME and SIN-1 caused significant TBF alterations (−44% and +136%, respectively) with less marked (−15% and +33%) alterations in normal brain. Calculated cerebrovascular resistance changes within tumour were indeed selective. Baseline TCP was increased compared with normal brain (20-fold). L-NAME and SIN-1 administration did not alter TCP. These effects have significant implications for human malignant glioma management. Selective i.v. manipulation of LCBF, without significant changes in TCP, could increase the efficacy of chemotherapy, radiotherapy or provide better peritumoural oedema control.