1. Two vasodilators, prazosin and hydrallazine, have been compared in three double-blind cross-over studies designed to test their effect when used in combination with a β-adrenoceptor-blocking agent and a thiazide.

2. Single doses of 3 mg of prazosin or 75 mg of hydrallazine were administered to patients whose blood pressures remained uncontrolled on a thiazide and a β-adrenoceptor-blocking agent. Both agents produced significant falls in systolic and diastolic blood pressure apparent at 1 h. The effects of prazosin persisted for 6–7 h and those of hydrallazine for 4–6 h. Tachycardia was more marked and more prolonged after hydrallazine and continued after the blood pressure had risen to base-line levels or above.

3. In 6 week and 12 week double-blind cross-over studies, mean falls in blood pressure were similar with prazosin and hydrallazine. Similar falls in the supine diastolic blood pressure were achieved with 1 mg of prazosin and 20 mg of hydrallazine, but for a given fall in supine diastolic blood pressure, prazosin produced a significantly lower standing diastolic blood pressure.

4. Severe side effects were more pronounced after hydrallazine, which necessitated withdrawal of seven patients, whereas only one patient on prazosin withdrew from the trial because of side effects.

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