The influence of short-term hypertension on the haemodynamic response to exercise was studied by measuring arterial pressure and regional blood flow (radio-active microspheres) in dogs at two levels of treadmill exercise (5 km/h at 0 degrees and 10 km/h at 4.5 degrees). One group of dogs (n = 5) had one-kidney, one clip hypertension of 1-week duration and was compared to normotensive controls (n = 5). Both groups exhibited similar increases in heart rate, but arterial pressure increased only in the normotensive dogs. Left ventricular myocardial blood flow increased significantly in both groups. However, the increase was more pronounced in the hypertensive dogs. In some groups of active skeletal muscles (biceps brachii, tibialis cranialis), increases in blood flow in excess of 30-fold were observed during exercise in normotensive animals. Several muscles showed exaggerated increases in flow in hypertensive dogs. Perfusion of the brain, the kidneys and the splanchnic area was not compromised in either group of dogs. We conclude that in one-kidney, one clip hypertension of 1-week duration, when structural cardiovascular changes are presumably far from being completely developed, vasodilatation to increased metabolic demands in the myocardium and in most skeletal muscles is not limited in response to mild to moderate levels of exercise, and that perfusion of other vascular beds is not compromised.