The aim of this study was to assess the diurnal variability of heart rate during VT. For the purpose of this investigation VT was considered to be a minimum of 3 consecutive ventricular beats in duration at a rate more than 100 bpm. From the group of 287 patients with VT during 24-hour ECG monitoring, a selection was made of 52 patients in whom episodes of monomorphic VT occurred in the day-time and night-time without any changes of the QRS morphology. Thirty one patients had ischemic heart disease, 10--dilated cardiomyopathy, 2--mitral valve prolapse and 10 patients had no evidence of heart disease. In these patients the rate of VT (HR-V), basic heart rate (HR-S) before VT, and coupling interval (CI) of VT initiating beat were measured during day-time and night-time. The mean VT rate was 170 +/- 34 bpm during day-time and 149 +/- 36 bpm during night-time (p < 0.001). The mean sinus rhythm rate was significantly (p < 0.001) greater during day-time (88 +/- 16 bpm) than at night (78 +/- 19 bpm). There was significant difference in the mean values of the CI between day-time and night-time (504 +/- 122 vs 589 +/- 181 ms). A significant correlation was noted between HR-V and HR-S at night (r = 0.73; p < 0.001) but not during day-time (r = 0.38). Thus, HR-V similarly as HR-S is greater during day-time than during night-time. Diurnal variability of HR-V may be related to changes in autonomic nervous system tone.