Capillary diameter changes were studied in the tenuissimus muscle of 29 urethan-anesthetized New Zealand White rabbits. Capillaries were visualized with transillumination bright-field microscopy (saltwater lens, x 50; resolution approximately 0.3 microns). Median capillary diameter during the control period was 4.4 microns (range 3.2-6.9 microns). Complete aortic occlusion resulted in a reduction of median femoral arterial pressure to 17 mmHg (range 4-22 mmHg). During 2 min of occlusion, capillary diameter decreased by 6%, with greater change on the arteriolar side of the capillary than on the venular side. During reactive hyperemia after release of the occluder, capillary diameter maximally increased by 12% compared with the control period, with a larger response at the arteriolar end of the capillary than at the venular end. Median capillary resistance was estimated to increase by 27% during occlusion and to decrease by 36% during peak reactive hyperemia. The observed diameter changes are compatible with the idea that capillaries change their diameter relative to changes in transmural pressure.