Serum magnesium, calcium, sodium and potassium, and erythrocyte magnesium, sodium and potassium levels were measured in a selected group of 296 urbanised black male labourers in Johannesburg. Of these, 214 subjects were normotensive and 82 were hypertensive. The aim of the study was to examine the relationship between serum and erythrocyte electrolytes, specifically magnesium and calcium, and blood pressure in this population sample. The results showed a significant decrease in serum magnesium, calcium and potassium in the hypertensive subjects. Red blood cell magnesium was significantly lowered and red blood cell sodium was significantly raised in this group. A significant inverse correlation was found between serum magnesium and blood pressure, erythrocyte magnesium and blood pressure, serum calcium and blood pressure, and serum potassium and blood pressure. A positive correlation was observed between erythrocyte sodium and blood pressure. Of all the electrolytes assessed, magnesium (serum and erythrocyte) correlated most closely with blood pressure. The data reported here suggest that body magnesium and its interactions with calcium, sodium and potassium may, in certain groups of individuals, play an important role in the development and maintenance of high blood pressure.