To investigate circadian variation of cardiac arrhythmias ambulatory electrocardiogram monitoring was carried out before and after one week's treatment with a long acting beta-adrenoceptor blocker, nadolol, in 26 patients who presented with symptoms attributable to arrhythmias. Analysis of the 24 hour profile of premature ventricular contractions showed a significantly (P less than 0.05) higher frequency during midday to midnight than between midnight and midday. The frequency of supraventricular tachycardia was significantly (P less than 0.05) higher during the periods from 12.00 hours to 16.00 hours (11.0 +/- 12) and 16.00 hours to 20.00 hours (11.3 +/- 11) than during the periods of 00.00 hours to 04.00 hours (3.6 +/- 3) and 04.00 hours to 08.00 hours (6.0 +/- 8). The period of the highest incidence of all arrhythmias was between 16.00 hours to 24.00 hours, and that of lowest during the period between 04.00 hours to 12.00 hours (P less than 0.01). After one week's treatment with nadolol the frequency of all arrhythmias was strikingly reduced but their pattern of occurrence remained unchanged. These studies suggest that patients who present with symptoms attributable to arrhythmias tend to have these more frequently during the physical and mental activities of the day and evening presumably due to the accompanying sympathetic overactivity.