We evaluated the effect of angiotensin 1-type receptor blocker losartan (Los) on blood pressure (BP), heart weight/body weight (HW/BW) and geometry of thoracic aorta (AT) and carotid artery (AC) in spontaneously hypertensive (SHR) and Wistar rats. Los was administered in a dose 10 mg kg-1 day (1 b. w. by gavage from 4-week to 9-week of age. BP was measured by the tail plethysmographic method. After anaesthesia cardiovascular system was perfused with glutaraldehyde fixative (120 mm Hg) and AC, AT were processed for electron microscopy. In SHR, in comparison to Wistar rats, we observed increase of BP and cardiac hypertrophy (HW/BW); in both AC and AT increased wall thickness (WT), cross sectional area (CSA) and wall thickness/inner diameter (WD). Administration of Los to SHR decreased BP and HW/BW and WD of both AC and AT; ID of AC, AT was increased and WT of AC was decreased. Los administered to Wistar rats decreased BP, HW/BW and WT, CSA, WD of both AC and AT; ID of AC was increased. In summary: Los administration resulted in reduction of (i) blood pressure and (ii) HW/BW and (iii) remodelling of arterial wall of AC and AT of both SHR and Wistar rats. The study was supported by VEGA grant 2/3145/23, Slovakia.