Isolated rabbit anterior mesenteric–portal veins (A.M.V.) which possess vasomotion were perfused with Kreb's solution in an apparatus designed so that intraluminal pressure and longitudinal tension could be measured simultaneously. The rate of vasomotion increased as perfusion pressure was increased from 0 to approximately 5 or 6 mm Hg. The amplitude of these spontaneous contractions increased to a maximum at a perfusion pressure of approximately 6 mm Hg and then decreased as perfusion pressure was raised further. Noradrenaline (10−7 g/ml) increased the longitudinal tension, but slightly decreased intraluminal pressure. Isopropylnoradrenaline (10−7 g/ml) had little effect on intraluminal pressure but decreased the amplitude of spontaneous contractions. It is suggested that the effect of perfusion pressure on the frequency and amplitude of vasomotion in the A.M.V. is related to autoregulation and that this perfused preparation may be a useful model for study of the rheology and responses to drugs of the splanchnic circulation.