The effects of repetitive electrical stimulation of a lumbar sympathetic trunk (LST) for 30 s at various frequencies and supramaximum intensity on the nerve blood flow in a sciatic nerve were studied by laser Doppler flowmetry in anesthetized Fischer-344 male rats. The response was biphasic; i.e. an initial increase and then a decrease. The maximum mean increase after 2 Hz stimulation was 22 +/- 8%, while the maximum mean decrease after 20-50 Hz stimulation was 79 +/- 3%, of the prestimulus control level. The initial increase, which was greater at lower frequencies and existed even after local sympathetic denervation, was passive, and was caused by the systemic pressor response to LST stimulation. The decrease, which was nearly abolished by an i.v. alpha-adrenergic blocker, phentolamine (10 mg/kg), resulted from vasoconstriction in the vasa nervorum, mainly via activation of alpha-adrenergic receptors.