Introduction. Middle cerebral arteries (MCA) from adult spontaneously hypertensive rats (SHR) show a reduction in internal diameter (ID), which can jeopardize cerebral blood flow. Aims.To determine if ID reduction in MCA from adult SHR: 1)is due to a structural modification, an increased smooth muscle cell (SMC) contractility, or a defect in basal endothelial vasodilators and 2)if this ID reduction precedes or develops with hypertension.Methods.30-day old and 6-month old Wistar Kyoto rats (WKY) and SHR were used. Systolic blood pressure (SBP) was measured via the iliac artery in anaesthetised rats (sodium pentobarbital 50mg/kg i.p). Thereafter the rats were killed with pentobarbital. The investigation conformed with EU guidelines. MCA IDs were measured with pressure myography with and without Ca2+ (20-120 mmHg). Intrinsic tone and responses to 75 mM KCl, 10-4 M L-NAME (NOS inhibitor), 3×10-7M indomethacine (COX inhibitor), 10-8-10-5M sodium nitroprusside (SNP, NO donor) were studied at 70 mmHg. To determine NO and O2- availability, intact MCA segments were incubated with the fluorescent indicators diaminofluoresceine (105M, Em 550nm) and dihidroethidium (10-6M, EM 610nm) respectively, visualized with confocal microscopy and quantified by image analysis. Data were analyzed by 1 or 2-way ANOVA and are shown as mean ± SEM.Results.In 30-day old rats SBP was similar in WKY (89±4 mm of Hg n= 19) and SHR (100±4 n= 21, p=0.6 ) and it was significantly larger in adult SHR (183±10 n=17 p<0,05), when compared to age-matched WKY (127±10 n=12). In young SHR and WKY rats MCA ID was similar at all pressures tested in the presence or absence of Ca2+. MCA ID was significantly smaller in adult SHR the presence of Ca2+ at every pressure tested (at 70 mm Hg IDSHR=108±11, IDWKY=154±8, p<0,01) This difference was abolished by 0Ca2. Intrinsic tone was significantly larger in SHR MCA.In adult rats KCl response was similar between strains, suggesting no changes in SMC contractility. Indomethacine had no effect on ID, discarding a prostacyclin defect. L-NAME significantly reduced ID in basal conditions and abolished the differences in ID between strains (at 70 mm Hg IDSHR=186±4, IDWKY=200±10, p=0.22) suggesting a defect in NO pathway. In the presence of L-NAME, to exclude endogenous NO, SNP relaxation was significantly smaller in adult, but not in 30 day-old, SHR rats. MCAs from adult SHR showed an increase in NO and a decrease in O2- availability. Conclusions.In MCA from adult SHR: 1) an increased intrinsic tone due to a defect in NO-cGMP pathway is responsible for the reduction in ID, 2) an increased NO release may act as a mechanism to compensate for the reduced cGMP-mediated responses, and 3) these alterations are due to the effect of hypertension. Supported by MCyT (BFI 2001-0638).