L-Alanyl-L-proline-d3 t-Butyl Ester Hydrochloride is an intermediate in synthesizing Enalapril-d3 Hydrochloride (E555258), which is the hydrochloride salt of Enalapril-d3 (E555253), a labeled version of Enalapril (E555403), an antihypertensive that is also an angiotensin-converting enzyme (ACE) inhibitor.