1. Prazosin had a considerable anti-hypertensive effect in both lying and standing posture in a mixed group of twenty-four patients in an open clinical trial. The drug was well tolerated and side-effects were few.
2. Tested on rat blood vessels, prazosin was ten times more potent on a molar basis than phentolamine in blocking the vasoconstrictor effects of noradrenaline. In the absence of vasoconstrictor nervous activity, no vasodilatation was observed.
3. In genetically hypertensive rats, prazosin in large doses caused a substantial fall in blood pressure, total exchangeable sodium and extracellular fluid volume. Tolerance to these effects started to develop within 20 days. In normotensive rats, blood pressure was lowered but total exchangeable sodium and extracellular fluid volume were not affected.