We monitored, by the Holter method, 23 clinically stable maintenance hemodialysis patients for 5 +/- (SEM) 2 hours before hemodialysis, 5.0 +/- 0.5 hours during hemodialysis, and 13 +/- 3 hours after hemodialysis. Of 23 patients, 9 (39%) had unexpected frequent or complex ventricular arrhythmias recorded and after hemodialysis with a potassium dialysate bath concentration of 2.0 mEq/liter. Patients with ventricular arrhythmias were more likely to be using digoxin (8/9 vs. 1/4) and to have evidence of left ventricular hypertrophy (9/9 vs. 7/4 than were those patients without arrhythmias. Of these 9 patients with arrhythmias, 6 underwent repeat Holter monitoring during multiple dialysate protocols. Of the 6 patients, 4 had a significant reduction in the frequency of ventricular ectopy when a dialysate of 3.5 mEq/liter potassium was used (P < 0.05), but of these 6, 3 still had complex arrhythmias. The use, however, of a 3.5 mEq/liter potassium dialysate plus the administration of a 400-mg dose of quinidine sulfate orally 45 min prior to hemodialysis was successful in reducing ventricular ectopic frequency and complexity in all the patients studied. Conclusion. Maintenance hemodialysis patient using digoxin and with left ventricular hypertrophy have an unexpectdly high indicence of occult, potentoial serious, ventricular arrhythmias during and after hemodialysis, revealed by Holter monitoring. There is preliminary evidence that a low-potassium bath concentration may play a role in predisoposing patients to these arrhythmias. Further prospective studies with largaer number of patients will be needed, however, to evaluate the significance of these findings.