Teprotide, a nonapeptide isolated from the venom of a Brazilian pit viper, Bothrops jararaca, was the first angiotensin converting enzyme (ACE) inhibitor to be discovered and tested. It was found to be an effective, non-toxic antihypertensive agent as well as an afterload-reducing agent for patients with congestive heart failure (CHF). The primary activity of teprotide resulted from blockade of the angiotensin I converting enzyme--the pivotal step in the renin-angiotensin-aldosterone system (RAAS), and consequent reductions in angiotensin II levels. There was limited clinical testing for teprotide because of: its scarcity; the need for parenteral administration; and the subsequent discovery and synthesis of captopril, the first orally active angiotensin converting enzyme inhibitor.