The longitudinal musculature of the rat portal vein is characterized by spontancous, rhythmic contractions indicating effective intercellular conduction in this vascular smooth muscle. The present study was done in order to evaluate the relative importance of neurogenic and myogrnic mechanisms for the spread of excitation in an isolated preparation of this vessel. A local anesthetic agent, in concentrations which blocked impulse transmission in autonomic nerves, failed to interfere with conduction in the smooth muscle of the portal vein whereas hyperosmolarity. which is considered to preclude electro tonic spread of action potentials between muscle cells (Barr. Dewey and Rerger 1965), caused desynchronization of the contractile activity. It is concluded that conduction in this vascular smooth muscle is myogenic in nature. The fact that the portal vein, in contrast to most other vessels, is equipped with a “double plexus” of adrenergic nerve fibres (Fuxe. personal communication) does not seem to be important for coordination of its spontaneous contractions. The influence of the hyperosmotic environment on the electrical and mechanical activity of the portal vein is illustrated by experiments with a sucrose-gap technique. The mode of action of hyperosmotic solutions is discussed.