The present study investigates the question of capillary recruitment and reserve capillaries in the brains of awake rats. Perfused capillaries were marked by intravenous globulin-coupled fluorescein isothiocyanate (FITC) and cerebral blood flow was measured autoradiographically. During hypercapnia, the density of perfused capillaries was unchanged compared to normocapnia, although blood flow was markedly increased. This shows the lack of capillary recruitment in the brain during the high flow that occurs during hypercapnia. In additional studies using fluorescent staining both of morphologically existing and of perfused capillaries, perfusion of all capillaries during normal, normocapnic conditions was found. These experiments show the lack of any capillary reserve in the rat brain.