The carboxylic acid group on the proline of 1 was replaced by a phosphoric acid, a hydroxamic acid, and a tetrazole to give compounds 2-4, respectively. Testing of 2-4 as angiotensin converting enzyme (ACE) inhibitors gave I50 values of 100, 1.6, and 22 microM, respectively, compared to 0.07 microM for 1. A hydroxamic acid derivative of the ketomethylene pentapeptide analogue 18 was then synthesized. This compound, 17, had an ACE I50 of 0.011 microM compared to 0.0076 microM for 18. Oral administration of 10 mg/kg of 17 to renal hypertensive rats had no effect on blood pressure or heart rate.