Sixteen subjects, mean age 59 +/- 18, 11 normal and 5 with coronary artery disease, all having premature ventricular and/or atrial beats in the standard resting electrocardiogram (ECG), were studied to analyse the chronobiologic parameters of these arrhythmias. Single cosinor analysis of the data obtained by 96-hour ECG performed according to the Holter system, demonstrated: A) significant circadian rhythm in heart rate for all the subjects, with acrophases occurring between 12.56 and 17.36; B) significant circadian rhythms of premature ventricular beats for the majority of the subjects, with acrophases distributed along the 24 hours; C) significant circadian rhythms in premature atrial beats for 8 subjects, with acrophases occurring between 04.24 and 18.12; D) a spectrum of significant ultradian rhythms in heart rate with various periodicities, both in normal and in coronary patients; E) significant ultradian rhythms in premature ventricular beats for 8 subjects with periods ranging from 5h 15' to 17h. Population mean cosinor analysis demonstrated: A) significant circadian group rhythm in heart rate for all the 16 subjects and for the group of 11 normal subjects; B) no significant circadian group rhythm for premature ventricular and atrial beats. These findings suggest that the study of the individual chronobiologic pattern of premature beats may help to optimize antiarrhythmic therapy.