Experiments were carried out on 8 healthy ponies to examine the effects of prolonged submaximal exercise on regional distribution of brain blood flow. Brain blood flow was ascertained by use of 15-microns-diameter radionuclide-labeled microspheres injected into the left ventricle. The reference blood was withdrawn from the thoracic aorta at a constant rate of 21.0 ml/min. Hemodynamic data were obtained with the ponies at rest (control), and at 5, 15, and 26 minutes of exercise performed at a speed setting of 13 mph on a treadmill with a fixed incline of 7%. Exercise lasted for 30 minutes and was carried out at an ambient temperature of 20 C. Heart rate, mean arterial pressure, and core temperature increased significantly with exercise. With the ponies at rest, a marked heterogeneity of perfusion was observed within the brain; the cerebral, as well as cerebellar gray matter, had greater blood flow than in the respective white matter, and a gradually decreasing gradient of blood flow existed from thalamus-hypothalamus to medulla. This pattern of perfusion heterogeneity was preserved during exercise. Regional brain blood flow at 5 and 15 minutes of exercise remained similar to resting values. However, at 26 minutes of exercise, vasoconstriction resulted in a significant reduction in blood flow to all cerebral and brain-stem regions. In the cerebellum, the gray matter blood flow and vascular resistance remained near control values even at 26 minutes of exercise. Vasoconstriction in various regions of the cerebrum and brainstem at 26 minutes of exertion may have occurred in response to exercise-induced hypocapnia, arterial hypertension, and/or sympathetic neural activation.