The effects of five different dietary levels of vitamin D3 on the coronary arteries of groups of 17-60 2-month old weanling Yorkshire swine were studied. Four groups were fed the following levels of vitamin D3 per ton of ration continuously for 4 months: group I--100,000 IU, group II--300,000 IU, group IV--2,000,000, and group V--4,000,000 IU. Swine in group III were fed the diet containing 100,000 IU of vitamin D3 per ton of ration for the first 2 months of the study after which they were fed a diet supplemented with 4,000,000 IU of vitamin D3 per ton of ration for the remaining 2 months of the study. The highest degree of intimal thickening of the coronary arteries was observed among group V. The thickened areas contained numerous lipid-containing cells and degenerate cells without stainable lipid. Electron microscopic examination revealed a greater frequency of degenerate cells without stainable lipid in the coronary arteries of groups III and IV than in groups I and II. These results suggest a possible link between excessive daily intake of vitamin D3 and the development of human coronary atherosclerosis.