OBJECTIVE: Autonomic nervous system function in patients with diabetes mellitus (DM), especially those with diabetic autonomic neuropathy (DAN), displays an abnormal circadian pattern compared to normal subjects; this probably plays an important role in the onset of acute cardiovascular syndromes, which display a similar pattern of occurrence with a blunted late morning peak, and an increase of episodes during the night, in comparison to non-diabetic subjects. This study was undertaken to investigate the effect of an angiotensin-converting enzyme inhibitor, quinapril, on the circadian pattern of heart rate variability (HRV), a reliable index of sympathovagal interactions, in patients with definite DAN. METHODS & RESULTS: Normalised HRV frequency domain indices [high frequency power (HFP), reflecting vagal tone, low frequency power (LFP), reflecting both vagal and sympathetic (predominantly) modulation, and their ratio (LFP/HFP), indicative of sympathovagal balance] were assessed in 60 patients with DAN at baseline and one year after therapy with quinapril (n = 30), or placebo (n = 30) on a 24-hour 2-channel electrocardiogram with a solid state Holter monitor. Normal subjects (n = 30) and patients with DM without DAN (n = 30), were used as controls. The baseline circadian variation of fractional normalised power in DAN patients was abolished, with pronounced dominance of LFP over HFP during the whole 24-hour period. After one year of treatment, quinapril increased HFP, decreased LFP and improved their ratio, in the morning (07.00 a.m. to 15.00 p.m.) and night (23.00 p.m. to 07.00 a.m.) time intervals, with maximal effect in the night time interval (HFP = 20%, LFP = -8%, LFP/HFP = -31%; for all comparisons p < 0.05 vs baseline values and p < 0.001 vs one year of placebo). CONCLUSIONS: Quinapril increased HFP and decreased LFP as well as their ratio, all indicative of sympathetic predominance reduction, in patients with DAN at time intervals these indices were most adversely affected (morning and night). Since autonomic function is an important contributor in the pathogenesis of acute coronary events, malignant arrhythmias and sudden cardiac death, improvement of indices related to autonomic function in DAN patients in these time intervals may prove beneficial in clinical practice.