BACKGROUND: The role of nitric oxide (NO) in regulation of systemic blood pressure both in normotensives and in hypertensives remains unknown. OBJECTIVE: To investigate the influence of L-arginine (the substrate for generation of NO) on blood pressure in 30 men (aged 51.5 +/- 7.3 years) with established primary arterial hypertension that was not well controlled. METHODS: The antihypertensive therapy was not discontinued and the patients were administered beta-blockers and calcium antagonists only. On day '0' the patients were administered 250 ml 0.9% NaCl during 180 min intravenously. Then we infused 250 mg/kg L-arginine diluted in 250 ml 0.9% NaCl over 180 min into the antecubital vein for four consecutive days (days 'I'-'IV'). Conventional blood pressure and heart rate measurementws were performed before infusion and every 30 min during infusion. Twenty-four-hour ambulatory blood pressure monitoring (with a SpaceLabs 90207 device) with half-hourly recordings during the daytime and during the night-time was performed on the day of NaCl infusion ('0') and repeated on the day of L-arginine infusion ('II') for all patients. As indicators of generation of NO, blood level of cyclic GMP and urinary concentration of nitrite/nitrate were measured. RESULTS: On all days of L-arginine infusion we found significant falls in systolic and diastolic blood pressures (P < 0.0001) with an accompanying significant increase in heart rate (P < 0.001). The most potent hypertensive effect during infusion of L-arginine was observed on day 'I'. Mean systolic blood pressure decreawsed from 152.1 +/- 20.0 mmHg to a minimum of 123.3 +/- 16.2 mmHg after 60 min of the infusion (by about 19%). The maximal percentage fall in systolic blood pressure was consecutively lower on each day oif L-arginine infusion. It was 14.3% on day 'II', 11.9% on day 'III' and 10.1% on day 'IV'. Similarly, the greatest reduction of diastolic blood pressure was observed during infusion of L-arginine on day 'I'. Mean diastolic blood pressure decreased from 95.9 +/- 13.6 to 80 +/- 9.7 mmHg after 120 min of infusion (by about 17%). On the consecutive days maximal falls in diastolic blood pressure compared with its initial value were 13.5% on day 'II', 10.4% on day 'III' and 9.8% on day 'IV'. Twenty-four-hour ambulatory blood pressure monitoring revealed a significant decrease in diastolic blood pressure during infusion of L-arginine compared with day 0 when 0.9% NaCl was infused. Systolic and diastolic blood pressure variabilities were significantly decreased and the day-night differences in systolic and diastolic blood pressures were increased after infusion of L-arginine. We found a significant correlation between heart rate and systolic blood pressure both during the daytime and during night-time on the day of infusion of L-arginine. An increase in urinary concentration of nitrite/nitrate was observed after administration of L-arginine.CONCLUSION: The present results demonstrate that infusion of L-arginine can influence the systemic blood pressure in hypertensives through NO synthesis. By using ambulatory blood pressure monitoring we documented that the hypotensive effect of L-arginine seems to be limited to the infusion period itself. A decrease in blood pressure variability might imply an increase in sensitivity of baroreceptors or an improvement of autonomic functioning.