The objective of the present study was to compare systemic and regional haemodynamics in a large series of spontaneously hypertensive rats (SHR, n=32) with normotensive Wistar-Kyoto rats (WKY, n=26) at the age of 12-16 weeks. All rats were anaesthetized with thiobutabarbital and the radioactively labelled microsphere method was used to evaluate regional blood flow with special emphasis on different cerebral areas. The high blood pressure in the SHR was mainly due to elevated total peripheral resistance, which was 90% higher in the SHR compared to the WKY. Furthermore, heart rate was 25% (p<0.001) higher, but the cardiac index was lower by 20% (p<0.01) in the SHR. Blood flow was significantly lower and vascular resistance higher in several organs such as the kidneys, other visceral organs, skeletal muscle and skin of the SHR compared to the WKY. On the contrary, blood flow in the myocardium was augmented by 40% (p<0.01) in the SHR. Blood flow was 20-50% higher in the cerebral cortex, thalamus and caudatus (p<0.05-0.001), but attenuated in the hypophysis of the SHR. In the pons, medulla and cerebellum, blood flow was similar in the two strains. In this large microsphere study, the basal cardiac index was lower in the SHR already at this relatively early stage of established hypertension. Despite this, increased blood flow in the above mentioned cerebral regions was found in the SHR compared to the WKY.