The number of perfused capillaries and their spatial distribution at various levels of vascular tone were examined by intravital microscopy in the rabbit tenuissimus muscle. An increase in oxygen availability (ambient pO2 raised) reduced the density of perfused capillaries in proportion to the pO2 increase. This was attributed to a graded increase in resistance in all terminal arterioles rather than to total closure of some of them. Reduction of perfusion pressure by gradual occlusion of the abdominal aorta resulted in a gradual decrease in the number of perfused capillaries in the pressure range below 50 mm Hg, despite a concomitant increase in terminal arteriolar diameters. Within a group of capillaries supplied by the same terminal arteriole, the perfusion response to changes in hindlimb arterial pressure in a passive vasodilated bed was similar to the response to active changes in arteriolar diameter, as induced by variations in ambient pO2. The data suggest the presence of a yield shear stress within the capillaries that has to be overcome by the perfusion pressure for flow to occur, and in accordance, that the number of perfused capillaries is a function of the pressure gradient over the capillary bed, actively determined by the resistance in the arteriolar section. Graded variations in this resistance will lead to graded changes in the density of perfused capillaries, implying that all-or-none behaviour of the terminal arterioles is not necessary for such variations to occur.