Twenty patients with essential hypertension were treated with guanfacine given in single 1-5 mg daily doses over a period of 24 weeks. Compared with the initial values at the end of the first washout period, there was a significant decrease in blood pressure and heart rate. The most common side effect, dryness of the mouth, usually disappeared after 8-10 weeks of treatment. No changes in laboratory values were seen. In the post-treatment placebo period there were significant increases in blood pressure and heart rate compared with the last readings during the treatment period. However, these never exceeded the pretreatment values. In a second series guanfacine (1-5 mg daily) was abruptly discontinued in 11 patients after 6-20 weeks of treatment. Blood pressure was measured twice a day, in lying and standing positions, during the 4 days preceding abrupt withdrawal of guanfacine and for 7 days after discontinuation. Clopamide was given concurrently to two patients, and this was continued after withdrawal of guanfacine. Only in two patients did the blood pressure rise to values above the initial levels (30 mmHg systolic and 10 diastolic), but no clinical symptoms were observed during the withdrawal. A transitory increase in heart rate between 10 and 30 bpm was observed in five patients after abrupt discontinuation of the drug.