The prevalence, characteristics and circadian pattern of silent myocardial ischemia, and its association with ventricular arrhythmias was studied in hypertensive men aged 35 to 70 years (mean 61) without clinical cardiac disease. Participants were withdrawn from diuretic treatment and received 1 month of oral electrolyte repletion with 40 mmol of potassium chloride, and 400 mg of magnesium oxide daily. Twenty-four-hour Holter monitoring was then performed. Episodes of silent myocardial ischemia occurred in 50 of 186 men (27%) and lasted from 2 to 289 minutes (mean 30 and median 18). Statistical analysis comparing the interval from midnight to 6 A.M. with each of the other three 6-hour time intervals revealed that participants were less likely to have silent myocardial ischemia in this period (p less than 0.01 for each comparison) than at other times of the day. There was little difference in the proportion of men with a frequent or complex ventricular arrhythmia during the entire day or within 1 hour of the silent myocardial ischemic episode (or during a comparable time period) comparing those with to those without silent myocardial ischemia. These findings indicate that silent myocardial ischemia occurs in approximately 25% of an older population of hypertensive men without history of symptomatic cardiac disease. The circadian pattern of frequency of silent ischemic events in men free of clinical cardiac disease is similar to that reported for patients with cardiac disease and coincides with that reported for sudden death. There was no significant association between silent myocardial ischemia and ventricular arrhythmias.