Circadian variations of sinus rate (SR) are well described in subjects with normal sinus node (SN). On the other hand there are no data in literature concerning the SR daily variations in patients with SN dysfunction. In order to clarify this problem we studied the SR circadian variation in healthy subjects (12) and in patients with intermittent bradycardia (11), persistent bradycardia (9) and intermittent 2:1 sino-atrial block (7). Sr was recorded utilizing ECG Holter monitoring. By histograms obtained every 2 h, maximum, modal, and minimum SR were evaluated. Time series were analyzed with a computer program for cosinor method. In the patients with normal SN, circadian periodicity in SR was confirmed with acrophase at 16(26) (Maximum SR), 16(38) (minimum SR) and 16(31) (SR mode), respectively. In patients with intermittent bradycardia a circadian rhythm was detected only in minimum SR (Acrophase: 16(51)) and in SR mode (acrophase: 16(13)). No statistically significant circadian variations of the 3 parameters of SR were observed in patients with both persistent bradycardia and sino-atrial block. These data suggest that: 1. the disappearance of the circadian periodicity is proportional to the severity of SN dysfunction; 2. the patients with intermittent bradycardia show a more normal behavior at low rates than at high ones; 3. the disappearance of circadian rhythm in patients with marked SN dysfunction can be related to the SN intrinsic involvement and/or autonomic nervous system dysfunction.