A novel sensitive and specific method for the measurement of tissue angiotensin-converting enzyme (ACE) activity utilizing HPLC is described. ACE activity was determined in detergent-extracted canine hearts utilizing the synthetic ACE-specific substrate hippuryl histidyl leucine (HHL), both in the presence and the absence of the site-specific inhibitor captopril. Tissue ACE activity was quantitated from the moles of hippuric acid (HA) formed, in time-fixed assays, utilizing HPLC separation of HA from HHL and UV-spectrophotometry for quantitation of HA as in the standard Cushman and Cheung assay (Cushman DW and Cheung HS, Biochem Pharmacol 20: 1637-1648, 1971). Separation of HA from HHL was performed by reverse phase HPLC on a phenyl silica gel column with an eluent consisting of 20% acetonitrile in 0.1 M aqueous ammonium phosphate buffer, pH 6.8. After the standard liquid/liquid extraction procedure with ethyl acetate, HPLC analysis revealed the presence of unreacted substrate, HHL, in amounts comparable to the product of interest, HA, in the final assay; moreover, the amount of HA formed did not fall completely to zero in the presence of captopril. Regional studies of canine cardiac ACE activity utilizing the HPLC-based assay and the standard assay method showed a significantly higher ACE activity in the right ventricle compared with the left ventricle (2.37 ± 0.7 vs 1.24 ±0.18 mU/g, P < 0.05 [N = 6], respectively) in the HPLC-based assay, but no difference in right and left ventricular ACE activities by the standard assay (0.25 ± 0.08 vs 0.31 ± 0.09 mU/g [N = 6], respectively). Kinetic studies utilizing the HPLC-based assay coupled with the use of captopril showed Km (1.34 ± 0.08 mM) and Vmax (36.8 ± 11.5 × 10-10 M/min) values in agreement with those in the literature. Our results demonstrate that the application of HPLC to the standard Cushman and Cheung assay improves the sensitivity and specificity of the standard assay and enables the use of much smaller amounts (~4 vs ~400 mg for the Cushman and Cheung assay) of tissue for ACE activity assay. © 1995.