The synthesis of a series of orally active, phosphinyloxyacyl proline inhibitors of angiotensin converting enzyme (ACE) is described. The in vitro and in vivo ACE inhibitory activities are reported for each compound. The structure-activity relationship for this series of compounds in relation to the carboxyalkyl dipeptide ACE inhibitors as well as other types of hydroxyphosphinyl-containing ACE inhibitors (e.g., the corresponding nitrogen and carbon isosteres) is discussed. Within an isosteric series of phosphorus-containing inhibitors based on the lysylproline terminal dipeptide sequence, only the phosphonates (oxygen isosteres) show a high level of oral activity. Optimum potency and oral activity in the phosphonate series occurs with the (phenylbutyl)- and n-hexylphosphonate side chains. An aminobutyl side chain in the P1' residue is an absolute requirement for full expression of oral activity. The most potent of these compounds, 8b (SQ 29,852), has intravenous and oral activities superior in potency to those of captopril in the normotensive rat.