OBJECTIVES: Presence of cardiovascular symptoms in perimenopausal period is suggested to be associated with the decrease of cardiac autonomic nervous system (cANS) regulatory function. AIM: Aim of the study was to evaluate whether menopause affects circadian heart rate variability (HRV). METHOD: 42 healthy women were divided into two equal groups: premenopausal (age 43 +/- 7 years) and postmenopausal (age 53 +/- 3 years, 3.25 +/- 1.6 years after the last menstruation). Menopausal status was verified by plasma estradiol and FSH activities. In both groups the outpatient 24-hrs ECG recordings were carried out. The HRV parameters (SDNN, pNN50, r-MSSD, nLF, nHF) were analysed within the day-time (6 a.m. - 10 p.m.) and the night-time (10 p.m. - 6 a.m.) ECG. RESULTS: Day-time recordings showed higher SDNN (128 +/- 23 vs. 104 +/- 36, p = 0.01), pNN50 (18 +/- 8.3 vs. 7 +/- 4.4 p < 0.001), nLF (74 +/- 11 vs. 33 +/- 5 p < 0.01) in the premenopausal patients and higher nHF in the postmenopausal women (39 +/- 3.4 vs. 26 +/- 11, p < 0.01). In the night-time recordings we observed higher pNN50 (24 +/- 20 vs. 13 +/- 10, p = 0.01) and nLF (66.5 +/- 17 vs. 37 +/- 5.8, p < 0.01) in the premenopausal patients with nHF higher in the postmenopausal group (44 +/- 5.7 vs. 31.4 +/- 17, p < 0.01). In the premenopausal women higher r-MSSD and nHF values were observed at the night time (respectively p = 0.03 and p = 0.04). Simultaneously higher night-time values of pNN50 (p = = 0.03) and nHF (p = 0.001) were noted in the postmenopausal group. CONCLUSIONS: Lower time HRV parameters suggest decreased parasympathetic regulation of the heart. Lower nLF indicates impairment of the arterial baroreceptor reflexes, whereas the increase of nHF requests further studies.