The long term effects of Mg on serum lipids were examined in uraemic rats. Experimental rats were randomized into two groups. Twenty four rats were made uraemic by 5/6 nephrectomy, and another 24 control rats were sham-operated. Both groups were then randomized into three subgroups. These three subgroups of uraemic and control rats received three different diets: a Mg-deficient diet, a moderately Mg-supplemented diet, and a highly Mg-supplemented diet. After 12 weeks on this regimen, all animals were killed. In uraemic rats, serum triglyceride and total cholesterol levels were higher than in control rats. These lipid levels were further increased in uraemic rats on Mg-deficient diet and were decreased in uraemic rats on highly Mg-supplemented diet. These results suggest that disturbance of lipid metabolism in uraemic rats is promoted by low Mg intake and is ameliorated by high Mg intake.