Prazosin, an orally active alpha-1 selective adrenergic antagonist, has been of value in treating patients with hypertension and congestive heart failure. In contrast to non-subtype-selective alpha-adrenergic antagonists and direct-acting vasodilators, prazosin's hypotensive action is accompanied by little or no increase in heart rate, plasma renin, or plasma norepinephrine. Prazosin is a versatile drug that may be used alone or in combination to treat mild, moderate, or severe hypertension. The antihypertensive effect is sustained, and may increase during long-term therapy. The major side effect, postural hypotension after the first drug administration, is related to drug dose and intravascular volume depletion. Other side effects are mild and seldom limit therapy. In patients with congestive heart failure, prazosin results in balanced venous and arterial dilation, similar to that produced by nitroprusside. Attenuation of some or all of prazosin's initial hemodynamic effects has been seen during multiple short-term administrations. However, chronic studies have shown sustained symptomatic and hemodynamic improvement during long-term administration; initial hemodynamic attenuation may be transient or partial, and does not preclude long-term effectiveness, particularly during exercise. Preliminary studies indicate that prazosin may also be effective in treating patients with peripheral vasospasm due to Raynaud's phenomenon or ergotamine overdose.