Vasoconstrictor and vasodilator neuroeffector transmission occurs in a variety of modes. Models are presented depicting vascular segments under the direct control of a single or multiple innervation, or an indirect influence via secondary release of a vasoactive substance. The neuroeffector relationship in the rabbit portal vein is described to illustrate the coexistence of several modes of transmission. ATP or its congener is released from the nonadrenergic vasodilator nerves in this vein, possibly as the transmitter. Further, a similar substance is released from the adrenergic vasoconstrictor nerves. It possibly acts directly on smooth muscle cells to relax them, and on the nerve terminals to inhibit the adrenergic transmitter release. Evidence suggests that such purine-mediated feedback mechanism may also occur at many other adrenergic and nonadrenergic neuroeffector synapses.