BACKGROUND: Heart rate variability (HRV) indices are accepted markers of cardiac autonomic activity and have been used as indicators of beta-blockade effects in congestive heart failure (CHF) patients. In view of the high frequency of sudden cardiac death in the morning, there is a question whether the evening beta-blocker administration would be more efficient than a morning dose. AIM: To compare HRV indices after morning or evening long-acting beta-blocker administration. METHODS: The study group consisted of 52 CHF patients (NYHA class II/III) in sinus rhythm. Time domain (TD) and frequency domain (FD) HRV analyses were performed for daytime, nighttime and a 24-hour period: first after the morning bisoprolol administration, and then after the same evening dose. RESULTS: After the evening dose the mean heart rate was significantly lower (p = 0.01), nighttime normal R-R intervals were significantly prolonged (p = 0.008) and the low frequency (LF)/high frequency (HF) ratio was significantly lower for: 24 h (p = 0.0002); daytime (p = 0.003) and nighttime (p = 0.008) with higher HF values in the 24-hour period (p = 0.0007) and in the daytime interval (p = 0.006). CONCLUSION: An evening dose of a beta-blocker is more effective than a morning dose in reversing adverse changes in the autonomic nervous system activity in CHF patients.