N alpha-(diphenoxyphosphoryl)-L-alanyl-L-proline, N alpha-[bis (4-nitrophenoxy)phosphoryl]-L-alanyl-L-proline, and N alpha-[ (2-phenylethyl)phenoxyphosphoryl]-L-alanyl-L-proline: releasers of potent inhibitors of angiotensin converting enzyme at physiological pH and temperature.