Abstract: AIM：To investigate the effects of angiotensin II type-1 receptor blocker on arterial myogenic tone of spontaneously hypertensive rats (SHR). METHODS: Sixteen SHR rats (8 weeks of age) were randomly divided into losartan 30 mg/(kg·day) group (SHR+L) and normal SHR group (SHR), and eight sex- and age-matched Wistar rats were assigned to control group (CON). Rats assigned to CON and SHR groups received vehicle (distilled water) by gavage, and distilled water containing losartan at 30 mg/(kg·day) was administered to rats in SHR+L groups. After 10 weeks, segments of middle cerebral artery were isolated and then cannulated to two pipettes. Vascular diameters in response to the increased intraluminal pressure (from 0 mmHg to 125 mmHg in 25-mmHg increments) of isolated middle cerebral arteries under no-flow conditions were recorded by a Pressure Myograph System both in physiological salt solution (PSS) (active diameter, Da) and calcium-free PSS (passive diameter, Dp). Myogenic tone was calculated by (Dp-Da)/Dp×100%. RESULTS: Myogenic tone responsiveness of middle cerebral artery in SHR group increased significantly compared with that in CON group, with 30 mg/(kg·day) losartan treatment in SHR+L group. This enhancement may be partially prevented. CONCLUSION: RAS (spell out) participates in arterial function remodeling in SHRs. Chronic AT1 receptor antagonist therapy with losartan markedly reduces the changes of function in SHRs.