In view of evidence suggesting an association of mild hypokalaemia with cardiac arrhythmia, the arrhythmogenic potentials of potassium losing and potassium sparing diuretic treatments were compared in a controlled prospective crossover study of 10 patients with mild hypertension and ischaemic heart disease. Mean (SEM) plasma potassium was 4.3(0.06) mmol/l and 3.3(0.07) mmol/l after potassium sparing and potassium losing treatments respectively. Blood pressure and volume depletion as assessed by weight change, plasma renin activity, and noradrenaline concentrations did not differ significantly in the two treatment periods. The potassium losing treatment phase was associated with an increased frequency of ventricular extrasystoles, a higher Lown grading during ambulatory electrocardiographic monitoring, prolonged duration and decreased phase 0 velocity of the monophasic action potential, a prolonged ventricular effective refractory period, and increased myocardial electrical instability as assessed by programmed ventricular stimulation. It is concluded that minor changes in plasma potassium concentration are associated with increased ventricular electrical instability in patients with ischaemic heart disease. Mild hypokalaemia in such patients may predispose to life threatening arrhythmias and should be avoided.