NADH fluorescence at tissue sites 15-20 microns in diameter and red blood cell velocity in adjacent capillaries were measured in resting sartorius muscle of the anesthetized cat during a 3-min period of sympathetic trunk stimulation. At stimulation frequencies of 2 and 4 Hz, red blood cell velocity fell briefly to 30-40% of control and then returned to approximately 75% of control values (vascular escape). No change in NADH fluorescence was observed. With stimulus frequencies of 6-12 Hz, flow reduction was greater and led to an increase in fluorescence when the flow reduction was > 50% and was sustained for > 30 s. NADH changes were more pronounced at tissue sites near venous capillaries than at sites near arterial capillaries. Hyperemia ensued after the end of sympathetic stimulation only when NADH fluorescence rose during stimulation. Resting blood flow in this muscle appears to be well above the minimum required to support oxidative metabolism. A shift to anaerobic metabolism does not appear to cause vascular escape during sympathetic stimulation but appears to be required for poststimulation hyperemia. These observations suggest that two separate oxygen-dependent mechanisms elicit vasodilation during and after sympathetic trunk stimulation.