Emergency treatment of acute, severe hypertension defined as diastolic blood pressure (DBP) greater than or equal to 135 mmHg combined with cerebral symptoms was prospectively monitored in a randomized multicenter study including 64 patients. Treatment was divided into two periods. In the first hour the patients were observed in the supine position after being given 40 mg furosemide intravenously. If DBP remained greater than 125 mmHg (n = 52), the patients were put on fractionated diazoxide administered intravenously (n = 28) or dihydralazine administered intramuscularly (n = 24). Blood pressure (BP) decreased with diazoxide from an average of 241/149 mmHg to 180/111 mmHg after 5 hours and with dihydralazine from 237/149 to 161/101 mmHg. The inter-individual BP response varied considerably. A clear and identical regression in neurological symptoms was observed on both drug regimens. No new neurological symptoms were seen to develop. It is concluded that a gradual fall in BP can be obtained after fractionated dosage of diazoxide (i.v.) as well as after dihydralazine (i.m.). The indication of acute parenteral therapy compared to less aggressive oral treatment is discussed.