OBJECTIVE: To describe the temporal patterns of heart rate and arrhythmias in the immediate postoperative period following cardiac surgery. Six postoperative cardiac surgical patients with a mean age of 48.3 years were studied. DESIGN: Descriptive longitudinal design. SETTING: Cardiac surgical ICU. METHODS: Heart rate and arrhythmias were recorded continuously for 48 hours from a cardiac monitor using a Holter tape recorder. Environmental and treatment data were noted throughout data collection by trained nonparticipant observers. RESULTS: Mean heart rate and incidence of arrhythmias were different between the 2 study days; therefore, data were divided into two segments (A and B). These differences coincided with extubation in most cases. Individual subject cosinor analysis revealed 24-hour rhythms of heart rate in both segments in all subjects except segment B for one subject. Rhythms of shorter periods were also found. In segment A individual subjects' acrophases (peak times of fitted curves) occurred later than expected for subjects' prehospitalization sleep-wake schedule, whereas in segment B they occurred earlier. Cosinor analysis of arrhythmias revealed significant 24-hour rhythms in both segments in one of the three subjects with premature atrial complexes, two of the four subjects with premature ventricular complexes and both subjects with ventricular couplets. Four-hour rhythms were found in premature atrial complexes (n = 1), atrial tachycardia (n = 1) and premature ventricular complexes (n = 3). Acrophases for arrhythmias varied among patients. During segment B the 4-hour-rhythm acrophases in heart rate and arrhythmias were related to the timing of respiratory therapy. CONCLUSIONS: Temporal variations in heart rate could be identified in these six critically ill adults. Rhythm parameters changed during the first 48 hours after cardiac surgery. In those who had arrhythmias, some patients demonstrated temporal patterns in the incidence of selected arrhythmias. Further study is needed to describe the temporal patterns of heart rate and arrhythmias in varied groups of critically ill persons in a variety of settings.