AIM: To characterize a reduction in heart rhythm variability (HRV) in patients with coronary heart disease (CHD), essential hypertension (EH) and in healthy controls during bicycle exercise test (BET). MATERIAL AND METHODS: BET was made in 21 CHD patients, 23 EH patients and 22 healthy controls (mean age 47.8 years, standard deviation 14 years). HRV was analysed before, during and after BET under continuous ECG monitoring (Cardiotens-01, Meditech, Hungary). Also, this device measured arterial pressure before and after exercise. BET (3 minutes of exercise plus 2 minute rest) was stepwise: the initial stage load was 25 W, at each subsequent stage the load was raised by 25 W RESULTS: In BET, lowering of HRV low-frequency power in CHD patients occurs at lower heart rate (HR) and load than in healthy controls and EH patients, it comes prior to ST wave depression. The individual index of exercise tolerance HRLF < 40/HRsubmax reflecting the ratio of HR at which LF < 40 ms2 is reached to submaximal HR in CHD patients is < 75%, in healthy controls > 80% irrespective of age or gender. CONCLUSION: CHD patients show lowering of HRV at lower HR than EH patients and healthy controls. The index HRLF < 40/HRsubmax under 75% can be used as an additional criterion of positive results of BET.