The effect of 7-nitro indazole (7-NI) and a range of substituted indazole derivatives on nitric oxide synthase (NOS) enzyme activity in homogenates of rat cerebellum, bovine endothelial cells and lung from endotoxin-pretreated rats was investigated. 3-bromo 7-nitro indazole was either equipotent (IC50, 0.86 +/- 0.05 microM c.f. 0.78 +/- 0.2 microM, n = 6, P > 0.05) or approximately 4x (IC50, 0.17 +/- 0.01 microM c.f. 0.71 +/- 0.01 microM, n = 6, P < 0.05) or 20x (IC50, 0.29 +/- 0.01 microM c.f. 5.8 +/- 0.4 microM, n = 6, P < 0.05) more potent than 7-NI as an inhibitor of bovine endothelial, rat cerebellar and rat lung NOS enzyme activity respectively. 2,7-dinitro indazole also inhibited NOS in all three tissue sources with a potency similar to that of 7-NI. These results suggest that 3-bromo 7-NI and 2,7-dinitro indazole may prove to be useful additional tools with which to examine the biological properties of nitric oxide (NO).