Although the immediate effects of angiotensin converting enzyme inhibitors (ACEI) on cardiac autonomic innervation have been extensively studied, little is known about circadian rhythm of heart rate variability (HRV) in these patients. The purpose of the present study was to investigate circadian changes of the autonomic activity in patients with essential hypertension treated with ACEI, which may play some role in the pathogenesis of hypertension. We studied 10 middle age patients with essential hypertension (treated with ACEI--enarenal at least 1 yr.) and 10 age, gender matched control subjects. Heart rate variability (HRV) was calculated from the 24-hr ambulatory electrocardiograms by the Fourier transformation. Power spectra were quantified at 0.003-0.04 Hz--very low frequency power (VLF), 0.04-0.15 Hz--low frequency power (LF) and 0.15-0.40 Hz--high frequency power (HF). The HF component and the ratio LF to HF were used as indices of the parasympathetic activity and balance between sympathetic and parasympathetic activities. RESULTS: The circadian rhythms of the LF, HF and LF/HF ratio differed significantly. In control group, LF, HF increased but LF/HF decreased (p < 0.05) at night, in hypertensive patients LF, HF increase was attenuated at night but LF/HF increased (p < 0.05). In control subjects when compared to hypertensive patients power spectrum HF was doubled at night. CONCLUSION: These findings suggest that in hypertension primary cardiac autonomic dysfunction might contribute to altered circadian rhythm leading to the development of cardiovascular diseases despite long term normalisation of high blood pressure.