Rats maintained for 12 weeks on diets moderately or more severely deficient in magnesium showed significant elevations in arterial blood pressure compared to control animals. Examination of the mesenteric microcirculation in situ revealed that dietary magnesium deficiency resulted in reduced capillary, postcapillary, and venular blood flow concomitant with reduced terminal arteriolar, precapillary sphincter, and venular lumen sizes. The greater the degree of dietary magnesium deficiency the greater the reductions in microvascular lumen sizes. These findings may provide a rationale for the etiology, as well as treatment, of some forms of hypertensive vascular disease.