This study was aimed to examine properties and changes in nitric oxide synthase (NOS) activity and cGMP level during reperfusion after 5 min of brain ischemia in gerbils. Animals were treated 5 min before ischemia with NOS inhibitors: N-Nitro-L-arginine (NNLA), or 7-Nitroindazole (7-NI), or with the inhibitor of guanylate cyclase, LY 83583, or with hydrocortisone for 7 days before ischemia. Northern blot analysis was performed using specific cDNA for inducible NOS. It was observed that ischemia significantly enhances NOS activity and cGMP level. During reperfusion, biphasic increase in NOS activity and cGMP level took place with two peaks 15 min and 2 h after ischemia. NNLA, 7-NI, and LY 83583 eliminated enhancements of NOS activity and cGMP level, whereas glucocorticoid remained without effect. There was no activation of gene encoding inducible NOS (iNOS). Our results indicate that ischemia-reperfusion activates constitutive NOS. It is suggested that nitric oxide (NO) production during reperfusion is related to neuronal degeneration and that inhibitor of NOS offers a new therapeutical strategies.