The work described in this thesis was conducted between October 1981 and September 1984 and is concerned with the synthesis of endophosphonopeptides as possible transition state surrogates. Various coupling methods for phosphonamide formation have been studied and several N-protecting functionalities evaluated in our synthetic strategy. The synthesis of an endophosphonoenkephalin an analogue of the naturally occurring pentapeptide leucine-enkephalin has been accomplished.