Twenty patients with uncontrolled hypertension were treated with pinacidil for a mean period of 43 weeks (range 10-63 weeks). All patients achieved and maintained significant reductions in blood pressure. The supine blood pressure at baseline was 184/116 mmHg; after one week it was 161/95 mmHg and at 43 weeks it was 138/79 mmHg. The mean dose of pinacidil was 30 mg/day. There was no significant difference between the two groups with respect to the dose of pinacidil or the blood pressure response. Pulse rate and weight remained stable for the group as a whole. Five patients were not taking beta-blockers. The mean baseline pulse rate for this group was 78 beats/min and when maintained on pinacidil it was 82 beat/min (NS). Six patients were not taking diuretics. The mean baseline weight for this group was 78.5 kg and while maintained on pinacidil it was 79.2 kg (NS). There was no occurrence of oedema, hirsutism or first dose phenomenon. The mean glomerular filtration rate and renal plasma flow for the renal plasma flow for the renal group was 35.4 ml/min and 192.3 ml/min before pinacidil and after six months they were 32.7 ml/min and 183.2 ml/min (NS) respectively. Six patients experienced minor side-effects. We conclude that pinacidil is a potent, well tolerated antihypertensive agent which merits further study.