Trimazosin was originated by Pfizer and was licensed to Bristol-Myers Squibb worldwide except for Canada, Mexico and the USA. Trimazosin is a quinazoline antihypertensive agent structurally related to the selective alpha 1-adrenoceptor blocker prazosin. Trimazosin is an alpha adrenergic receptor antagonist. Compared with prazosin, trimazosin was a less potent but more efficacious hypotensive agent. At doses which caused equal or even greater hypotensive effects than those caused by prazosin, trimazosin caused less inhibition of pressor responses to phenylephrine. When administered during a maximum hypotensive response to prazosin, trimazosin caused an additional fall in pressure. Trimazosin is an effective antihypertensive when given by itself or in combination with a diuretic. Its ability to induce vasodilation without concomitant sodium retention or stimulation of the renin axis may be an important factor in its effectiveness. Trimazosin has the potential to cause sustained improvement in left ventricular function, both at rest and during exercise, in patients with chronic congestive heart failure (CHF).