1. The relation between [Mg]o and the Mg content of the rat tail artery incubated in Ca‐free solutions was studied.2. Variation of [Mg]o in Ca‐free Krebs solution between 0·6 and 2·4 m m did not affect the cell Mg. In ATP‐depleted arteries, a sizeable fraction of the cell Mg was found to be proportional to [Mg]o.3. An even larger fraction of the cell Mg became proportional to [Mg]o in metabolically active arteries in which the transmembrane gradient of Na had been dissipated as a result of inhibition of the Na pump. In contrast to the extracellular Mg, the fraction responded to a change in [Mg]o at a very slow rate. The size of the fraction was reflected in net uptake of Mg if [Mg]o was higher than 1·2 m m. The rate of the uptake was lowered markedly by external Ca. All the Mg taken up by the cells was extruded again after restoration of the Na gradient.4. The uptake of Mg in Ca‐free, ouabain‐containing solution took place even if the concomitant swelling of cells was prevented by substitution of isethionate for external Cl.5. Under steady‐state conditions and at const. [Mg]o and [Na]o, [Mg]i increased with increasing [Na]i.6. The results are consistent with the hypothesis that the outwardly directed Mg pump in rat vascular smooth muscle utilizes the energy released in the course of spontaneous influx of Na.