The adrenergic neuroeffector mechanism has been assessed in 14 different veins of the rabbit chosen to represent vessels of different, function and regional location. The pattern, distribution and density of the adrenergic innervation were assessed by fluorescence histochemistry. The magnitude of the neurogenic contractile response of isolated segments was studied in vitro in relation to the maximum response to 1-norepinephrine. The diameter and wall thickness of the veins was determined. The amount of adrenergic innervation determined histochemically was related to the size of the contractile response. Both fluorescence and response were absent in the superficial cervical, femoral, subcutaneous from back, brachial and deep circumflex iliac branch veins. The amount of adrenergic innervation increased in order in the renal, inferior vena cava, pulmonary, jugular, parietal branch of internal iliac, mesenteric, central ear, cephalic and small saphenous veins. In the latter two veins, innervation was not restricted to the adventitio-medial junction but entered the media. The pattern of innervation and contractile response may be related to the functional need for active venoconstriction in the various parts of the capacitance section of the vascular system.