Intraluminal flow can cause both dilation or constriction of small arteries, depending on the level of tone. Both responses are specifically modulated in the same way by small changes in extracellular Na<sup>+</sup> ([Na<sup>+</sup>]<sub>e</sub>). We have investigated the effect of changes in [Na<sup>+</sup>]<sub>e</sub> on the level of contraction induced by a standard flow of physiological salt solution in ring of segments of the rabbit facial vein and on the associated <sup>45</sup>Ca<sup>2+</sup> influx and net uptake. Decreasing [Na<sup>+</sup>]<sub>e</sub> by 20% reduced the response to flow by 58%, and increasing it by 20% augmented the flow response by the same extent. There is a linear relationship between the level of the flow-induced contraction and [Na<sup>+</sup>]<sub>e</sub> over the range 120-180 m M. An alteration in [Na<sup>+</sup>]<sub>e</sub> of 10 m M corresponds to a 20% change in flow-induced contraction. This relationship is quantitatively the same as that previously reported for the rabbit ear resistance artery . Histamine (1 µ M) induced tone was not affected by changes in [Na<sup>+</sup>]<sub>e</sub> within the range 75-180 m M. <sup>45</sup>Ca<sup>2+</sup> influx and net uptake per unit force developed in response to flow were unaffected by these changes in [Na<sup>+</sup>]<sub>e</sub>. The flow-induced contraction of the vein was selectively, in comparison to that due to histamine, attenuated by amiloride, methyl-isobutyl-amiloride and monensin. It is argued that the observation that flow-induced contraction, caused by small alterations in extracellular sodium concentration, is not associated with changes in calcium uptake per unit force is consistent with an extracellular flow sensor. The similarities of the effects of sodium concentration change and amiloride, methyl-isobutyl-amiloride and monensin in the rabbit facial vein to what has been previously found in the resistance branch of the rabbit ear artery suggest that the flow-sensitive mechanism may be similar in the two vessels.