"Enkephalinase", a peptidase capable of degradating enkephalins, has been recently characterized in man, in both plasma and cerebro-spinal fluid (CSF). This study was designed to evaluate the ability of putative "enkephalinase" inhibitors, D-phenylalanine, captopril and thiorphan to decrease "enkephalinase" activity (EKA) in plasma and CSF in human sufferers. All drugs studied decreased plasma EKA. Captopril and thiorphan also decreased CSF EKA. Of the three drugs tested thiorphan proved to be the most potent "enkephalinase" inhibitor in both plasma and CSF. These results show the usefulness of EKA assessment as a procedure for evaluating the potency and specificity of putative "enkephalinase" inhibitors in man.