Magnesium and potassium are the 2 major intracellular cations. The intracellular concentrations of these 2 ions appear to be closely correlated, but the existence of a relationship between the plasma concentrations of these ions has been controversial. A major function of potassium is to maintain the excitability of nerve and muscle tissue. Alterations in either the intracellular or extracellular potassium concentration affect membrane excitability by alterations in the resting membrane potential. The critical factor is not the actual potassium concentration in either compartment but rather the ratio of the intracellular to the extracellular concentration. The intracellular concentration of potassium is maintained against an electrochemical gradient by active transport involving an ionic pump mechanism. In this metabolically active ion-pump mechanism the inward transport of potassium is balanced by the outward transport of sodium; this active transport mechanism involves magnesium. The plasma concentration of potassium has been reported to be an important factor in the genesis of cardiac arrhythmias. The arrhythmogenic mechanisms of hypokalaemia, particularly in mild degrees, have not been clearly defined. However, evidence has begun to accumulate that magnesium deficiency may be a critical factor in the cardiac arrhythmias associated with hypokalaemia. Diuretic drugs are recognised as primary agents in the treatment of essential hypertension. In patients on antihypertensive treatment evidence has recently been reported that there is a link between the administration of diuretics and sudden death. In addition to their action on the renal tubular handling of sodium and water, diuretic drugs affect the renal tubular handling of other ions. A well-established complication of therapy with diuretic drugs is an increased urinary excretion of potassium resulting in hypokalaemia. Hypokalaemia and hypomagnesaemia can be induced by the same mechanisms and are often clinically correlated with one another. The reported incidence of hypomagnesaemia is greater than that of hypokalaemia; a significant correlation also appears to exist between the plasma concentrations of magnesium and potassium. A significant inter-relationship between the plasma concentrations of magnesium and potassium and the evidence for a critical role of magnesium in the genesis of cardiac arrhythmias would support the proposal that magnesium should be routinely measured in situations, such as diuretic therapy, that are potentially associated with hypokalaemia.(ABSTRACT TRUNCATED AT 400 WORDS)