A sensitive radioimmunoassay (RIA) capable of measuring either lisinopril (1-[N2-[(S)-1-carboxy-3-phenylpropyl]-L-lysyl] -L-proline) or enalaprilat (1-[N-[(S)-1-carboxy-3-phenylpropyl]-L-alanyl] -L-proline), the active metabolite of enalapril has been developed. A suitable antiserum was raised against an immunogen prepared from conjugation of lisinopril, the lysyl analogue of enalapril, with succinoylated keyhole limpet hemocyanin. A novel radiotracer was also prepared for use in the assay by acylation of the epsilon amine group on the lysyl side chain of lisinopril with N-succinimidyl [2,3-3H]propionate. The antiserum was used at a final dilution of 1:44,500 and the sensitivity of the assay for enalaprilat was estimated at 2 pmol/mL plasma sample and 0.4 pmol/mL for lisinopril. Enalapril, the ethyl ester of enalaprilat, exhibited little cross-reactivity (0.005%), and several other compounds (captopril, proline, lysine, tyrosine, hippuric acid, and tryptophan) were found not to crossreact. In rabbits given a 2.03 mumol/kg iv dose of enalapril, plasma concentrations of enalaprilat were determined by the RIA technique and compared with an estimation of the enalaprilat concentrations derived from the extent of inhibition of plasma angiotensin converting enzyme (ACE). The plasma levels estimated by ACE inhibition were less than those obtained by the RIA in the first 45 min but were always greater in the samples taken after this time. Both assay methods showed that the conversion of enalapril to enalaprilat was rapid, and also indicated that there was initial rapid clearance of enalaprilat from the plasma.