BACKGROUND Patients with essential hypertension (EH) and left ventricular hypertrophy (LVH) have an increased risk of cardio-vascular complications. Alterations in the autonomic nervous system (ANS) activity may play a role in the development of serious cardiac arrhythmias and mortality in these patients. OBJECTIVE To examine the activity of ANS in patients with established EH in relation to the presence of LVH. METHODS The study group consisted of 70 subjects: 50 patients with untreated EH (mean age 44.4+/-12.7 years, 28 patients without LVH and 22 with LVH), and 20 age-matched healthy volunteers. ANS reactivity was assessed using heart rate variability (HRV) analysed during tilt table testing at 60 degrees. The following 5-min time-periods were analysed: (A) before tilting (supine position), (B) initial period of tilting, (C) last 5 min of tilting, and (D) immediately after completion of tilt test (after return to supine position). RESULTS Patients with EH without LVH had a higher increase of LF/HF values (period B versus period C) compared with controls (p<0.05). The change from tilt to supine position caused significantly lower change in lnHF values in patients with EH and LVH than in controls (p<0.01) or patients with EH without LVH (p<0.005). In patients with EH and LVH a significant correlation between HF and LVmass/height (r=-0.5, p<0.01) was noted. Compared with healthy controls, patients with EH and LVH had significantly lower HRV parameters [lnLF values were significantly lower in all analysed periods, (p<0.05), and lnHF - during period D, (p<0.01)] whereas LF/HF ratio assessed during period D was significantly higher (p<0.05). CONCLUSIONS In patients with EH without LVH a relative dominance of sympathetic activity is present. LVH in EH leads to a decrease in vagal drive and progressive inhibition of parasympathetic activity, both of which decrease HRV. These changes may play an important role in the electrical instability of hypertrophied myocardium.