The effects of magnesium (Mg) deficiency on the coronary arteries of 27 Yorkshire swine were studied by light and electron microscopy. The experimental animals were divided into 4 groups which received the following supplements: Group I, basal ration with adequate Mg (540 mg/kg diet), Group II, basal ration with insufficient Mg (270 mg/kg diet) Group III, 10% milk powder with adequate Mg (540 mg/kg diet), Group IV, 10% milk powder with insufficient Mg (270 mg/kg diet). Serum analysis indicated that dietary low Mg supplementation decreased cholesterol levels and increased phospholipid concentrations significantly. The highest magnitude and incidence of intimal thickening were observed in the coronary arteries of Group IV (p less than 0.003). No significant intimal thickening was detected in any of the other groups. Ultrastructural studies revealed a greater frequency of degenerated cells in Group III and IV (p less than 0.01). Numerous calcifications were observed in only Group IV. These data suggest that moderate Mg deficiency can promote atherosclerosis in combination with some atherogenic diet, and that the presence of smooth muscle cell degeneration is important in order for a magnesium deficiency to exert an effect on the coronary artery of swine.