In light of the important role of magnesium (Mg) in vascular smooth muscle function, this study was undertaken to examine effects of dietary Mg deficiency on membrane potential (Em), sodium (Na) efflux and distribution, and total ion content in rat aorta. Rats were fed either Mg-sufficient or Mg-deficient diets for 30 days. The aortic arch was removed and a segment incubated in 22Na to monitor kinetics of Na efflux. Another segment was used for measurement of Em under control conditions and during ouabain suffusion. Total tissue water and ion content were also determined. Results of this work indicate that, in aorta from Mg-deficient rats, Em was less polarized and Na accumulated intracellularly. Although the Em in Mg-deficient aorta was not affected by ouabain, which suggested Na-K pump inhibition, Na efflux was greater in Mg-deficient aorta, suggesting increased passive permeability changes to Na as the mechanism responsible for these changes.