Magnesium level in bone, skeletal muscle, the erythrocyte, serum and serum ultrafiltrate were estimated in ten patients with advanced uraemia. Eight patients were found to have significantly low skeletal muscle magnesium, indicating significant deficit of the cation. However, there was no correlation between skeletal muscle magnesium and magnesium levels of the other tissues. Bone magnesium was normal in all patients as was erythrocyte magnesium. Serum magnesium level was variable, but 6 of the 8 patients with low skeletal muscle magnesium had normal magnesium level in their serum ultrafiltrate. Although clinical features that might be attributed to magnesium deficiency were frequently seen in the uraemic patients, there is no discernible difference in clinical pattern between patients with or without magnesium deficiency as shown by skeletal muscle magnesium estimation.