Experiments were performed to explore the influence of two rauwolfia alkaloid derivatives, rescinnamine and syrosingopine, on noradrenaline at the sympathetic nerve-endings by delineating the localization of noradrenaline in the walls of femoral artery and abdominal aorta of rats treated with these agents compared to reserpine, using the fluorescence histochemical technique. Noradrenaline-containing nerve fibers emitting an intense yellowish-green fluorescence were demonstrated circumjacent internally to the adventitial smooth muscle layer of the femoral artery in both normal rats and spontaneously hypertensive rats (SHR). In both normal rats and SHR, treatment with rescinnamine or syrosingopine in an oral dose of 5 mg/kg caused essentially the same degree of depletion of the noradrenaline fluorescence localized subjacent to the adventitial smooth musculature of the femoral artery, as did that with 5 mg/kg of reserpine. The noradrenaline fluorescence vanished completely following treatment of rats with 15 mg/kg of rescinnamine or 30 mg/kg of syrosingopine. Noradrenaline-containing nerve fibers showing yellowish-green fluorescence were observed distributed subjacent to the entire internal circumference of the adventitial smooth muscle layer of the abdominal aorta in both the normal rats and SHR. In response to oral administration of rescinnamine or syrosingopine in a dose of 5 mg/kg, both normal and SHR rats showed a diminution of the noradrenaline fluorescence in the region of adventitial smooth musculature of abdominal aorta equivalent to that evoked by 5 mg/kg of reserpine. Treatment with 15 mg/kg of rescinnamine or 30 mg/kg of syrosingopine resulted in a complete disappearance of noradrenaline fluorescence in that region.(ABSTRACT TRUNCATED AT 250 WORDS)