Samples of normal heart muscle from an area with soft drinking-water (Burnley) contained significantly more magnesium and potassium than samples from an area with hard drinking-water (Hull). Females from the soft-water area had significantly more iron and copper. Males from the soft-water area had significantly less manganese and slightly less water in the heart muscle. Males in the soft water areas had significantly lower magnesium/potassium ratios, and both sexes had significantly higher potassium/sodium ratios, than those in hard water areas. The relationship between potassium/sodium ratio and death rates from arteriosclerotic disease suggests that the optimum potassium/sodium ratio is 2.8--3.0, and deviations in both directions are associated with increases in the death-rate from ischaemic heart-disease.