The development of dual inhibitors of the two zinc metallopeptidases, neprilysin (neutral endopeptidase) and aminopeptidase N involved in the inactivation of the opioid peptides, enkephalins, represents an attractive physiological approach in the search for new analgesics devoid of the major drawbacks of morphine. Phosphinic compounds, corresponding to the general formula H(3)N(+)-CH(R(1))-P(O)(OH