For determination of levels of plasmatic inhibitor of ACE (angiotensin convertase) a simple method was used based on a combination of enzymatic reaction followed by an HPLC determination of its product. The inhibitor (e.g. enalaprilat) was at first separated from the biological material by deproteination (methanol). Then, an aliquot of the sample was added to the reaction mixture containing a commercial ACE enzyme, its specific substrate FAPGG (N-(3-[2-furyl]acryloyl)-Phe-Gly-Gly) and buffer (Tris--HCl, pH 7.5). Degree of inhibition of the conversion of this substrate to FAP (desGlyGlyFAPGG) by the inhibitor present in the sample is related to its amount by a simple dose-response relationship. The amount of the FAP was determined by an HPLC on a RP-18 column with an acetonitril--nonylamine buffer (pH 2.4, adjusted with phosphoric acid) as a mobile phase with detection at 305 nm. Alternatively, the activity of the endogenous ACE present in the plasma was measured. The substrate FAPGG was added to the plasmatic sample containing both the inhibitor and endogenous ACE (as the sample was not deproteinized in this case) and the reaction product was determined as above. Inhibitor concentration has been obtained from a dose--response curve expressing the interaction with inhibitor with an ACE enzyme.