Protective effect of a brief ischemia period performed prior to ischemia, e.g. preconditioning, has been described in rat brain. Several mechanisms have been proposed to explain such a delayed protective effect. The time frame required for induction of tolerance in brain suggests new proteins synthesis as a mechanism. Because of involvement of nitric oxide pathway in cerebral ischemia physiopathology, we have investigated its role as a trigger of preconditioning.Male Wistar rats (280-320 g), anaesthetised with chloral hydrate (300 mg/kg) were subjected to a brief ischemia period (3 minutes) induced by intra-luminal occlusion of right middle cerebral artery (MCA). These animals received intra-peritonally N ω -nitro-L-arginine methyl ester (L-NAME; 3 mg/kg) or saline, 1-hour prior to the brief ischemia. Sham animals were operated in same conditions without 3-min MCA occlusion, one hour after receiving L-NAME or saline. Three days later, all the animals (n = 5-7/group) were subjected to 1-hour MCA occlusion, with the same method. Following a 24-hour period of reperfusion animals were sacrificed and infarct volumes were quantified after cresyl violet staining.There was no significant difference in blood pressure and blood gases between the different group. Infarct volume in pre-conditioned rats (126 +/- 13 mm 3 ) was significantly (p<0.05) decreased by 35% in comparison with sham rats (194 +/- 12 mm 3 ). This protective effect of preconditioning was only apparent in cortical part of infarcted zone. The beneficial effect of preconditioning remained unchanged despite previous administration of L-NAME (129 +/- 26 mm 3 ) whereas L-NAME has no significant effect by itself on infarct volume (232 +/- 3 mm 3 ).These data suggest that nitric oxide pathway is not involved as a trigger of preconditioning in rat brain.