Whether sympathectomy and somatic denervation in muscle flaps increased microcirculatory flow in the short or long term, thus producing an effect similar to the delay phenomenon, which increases survival in transferred skin flaps, was determined. The rat cremaster muscle flap model was used for in vivo microscopy. In the left cremasters of 30 Sprague-Dawley rats, the genitofemoral nerve was divided and the proximal vessels were stripped of their adventitia. The muscle was not elevated. In each rat, the contralateral cremaster served as the control. The rats were assigned to one of five groups: no delay before observation, a 24-hour delay, a 48-hour delay, a 7-day delay, or a 14-day delay. After the delay, red blood cell velocity, vessel diameters, number of functional capillaries, and leukocyte-endothelial interactions were measured. Microvessel response to topical vasoactive substances was measured. Immediately after denervation, red blood cell velocity increased transiently (71 percent; p = 0.006). Main arterioles dilated (20 percent; p = 0.02) at 24 hours, and capillary perfusion increased 36 percent (p = 0.001) at 2 weeks. The microvessels had hyperactive responses to all vasoactive agents 2 weeks after denervation. These findings indicate that proximal sympathectomy with somatic denervation leads to a triphasic, dynamic response in the peripheral microcirculation of the cremaster muscle flap. An initial acute hyperadrenergic phase was followed by a nonadrenergic phase, with significant vasodilatation, and a sensitized phase, with increased capillary perfusion and hyperresponsiveness to vasoactive substances. This study shows that with minimal access to the cremaster muscle flap neurovascular pedicle and without changing the blood supply to the flap, significant hemodynamic improvements can be made in the peripheral microcirculation.