Tiodazosin is a newly developed antihypertensive agent, structurally related to prazosin. Prazosin and tiodazosin administrated intravenously to anesthetized rats, are equally effective hypotensive agents, but that the hypotensive potency of prazosin is greater than that of tiodazosin. However, chronic administration of equivalent doses of the two compounds for 25 and 52 days via the drinking water indicated approximately equivalent, sustained reductions in blood pressure. Furthermore, at the end of the 52-day chronic dosing period tiodazosin caused appreciably less alpha-adrenergic receptor antagonist activity than prazosin as assessed by the norepinephrine dose-pressor response profiles.