1. In a cross-over study between hydrallazine and prazosin in fifteen patients, also treated with the β-receptor-blocking agent propranolol, 1 mg of prazosin was found to be equipotent with 30 mg of hydrallazine. 2. All patients were known to tolerate hydrallazine. In one patient prazosin treatment was discontinued because of severe headache. In four patients minor, non-persistent initial side effects were found. These might have been avoided by using a smaller starting dose. One patient fainted on the second day of prazosin treatment, probably from a micturition syncope. 3. Prazosin may be used as an alternative to hydrallazine. 4. The final status of prazosin as an anti-hypertensive drug is uncertain.