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To evaluate the existence and reproducibility of a circadian rhythm of ventricular premature complexes (VPCs), 38 patients (mean age 57 [+ or -] 17 years) with [is greater than or equal to] 30 VPCs/hour were studied with 24-hour electrocardiogram Holter monitoring. Nineteen patients had coronary artery disease and 19 had structurally normal hearts. A second Holter electrocardiogram was recorded in all patients from 2 to 47 days (mean 11) after the first. Chronobiologic analysis was made by single and mean cosinor methods. A significant and similar circadian rhythm of VPCs was found in the total sample both on the first (mesor 399, acrophase at 15:08, p (Am J Cardiol 1990;66:1099-1106), Ventricular premature complexes (VPCs) are a type of arrhythmia (abnormal heart rhythm) in which a heart beat arising in the ventricles occurs earlier than the normal heart rhythm. VPCs were shown to occur in a circadian rhythm or 24-hour interval. Treatment of ventricular arrhythmias may be more effective if drug regimens are designed with consideration of this circadian rhythm. However, it remains unclear whether the circadian rhythm of VPCs is reproducible in individual patients. Hence, the existence and reproducibility of a circadian rhythm of VPCs were assessed in 36 patients with 30 or more VPCs per hour. The electrical activity of the heart was monitored over a 24-hour period by electrocardiography. Disease of the coronary arteries, the major blood vessels supplying the heart, was present in 19 patients, whereas the remaining 19 had no evidence of structural heart disease. A second electrocardiogram was taken between 2 and 47 days after the initial recording. When assessing the total sample, a circadian rhythm of VPCs was detected on both recordings of heart electrical activity, with the greatest frequency of VPCs occurring in the late morning and afternoon. However, when assessing individual cases, only 18 patients showed a reproducible circadian rhythm of VPCs on both days; the remaining 20 patients had no significant circadian rhythm on one or both days. Among the 18 patients with a reproducible circadian rhythm, the greatest frequency of VPCs occurred during the waking hours in 14 patients and during the night in 4 patients. The reproducibility of a circadian rhythm of VPCs was unrelated to gender, the presence of coronary heart disease, medical treatment, number of VPCs at the start of the study, or daily variations in VPCs. However, patients with a reproducible circadian rhythm of VPCs were older than patients without a reproducible circadian rhythm. These findings suggest that VPCs can occur in a reproducible circadian rhythm and this factor should be considered when planning therapy. (Consumer Summary produced by Reliance Medical Information, Inc.)