Monoxidine is a new antihypertensive agent that, as pharmacological studies show, reduces blood pressure by stimulating central presynaptic alpha-2-receptors. In this open multicenter trial, 141 ambulatory hypertensives were treated for 12 months with monoxidine. After a run-in placebo period the mean supine blood pressure was 172.7 +/- 15.0/103.2 +/- 6.0 mmHg. After an individual dose-adjustment phase of 3 weeks starting from a daily dose of 0.2 mg monoxidine, the mean blood pressure decreased to 150.7 +/- 13.4/87.5 +/- 5.5 mmHg. In 96 patients, the blood pressure reduction was achieved within 3 weeks at a dose of 0.2 mg, while 52 patients required a dose of 0.4 mg. Over the 12-month treatment period, no tolerance to the drug developed. In 137/141 cases (97%), the drug was well or very well tolerated. At the state of treatment dryness of mouth was observed in 12.9% and lassitude in 4.8%. On discontinuation of monoxidine, blood pressure gradually increased. All in all, monoxidine proved to be well tolerated, reliable and safe in the long-term treatment of hypertension.