Angiotensin (Ang) I converting enzyme (ACE) inhibitors represent a major advance in the treatment of congestive heart failure, and tissue, rather than circulating ACE, may be their major site of action. However, assessments of tissue ACE inhibition in treated patients has not always supported this contention. In these studies, ACE activity was measured in homogenates of sampled tissue by biochemical methods. In the present study, using a model system, we have examined the validity of these tissue-sampling methods. Functional ACE activity was determined by comparing positive inotropic responses to [Pro10]Ang I in either vehicle-pretreated or ACE inhibitor-pretreated papillary muscles. [Pro10]Ang I elicits a response, which is entirely dependent on ACE-mediated conversion to Ang II. The ACE inhibitors studied were captopril, enalaprilat, lisinopril, and quinaprilat. In a parallel study, papillary muscle ACE activity was also measured in homogenates using [125I]MK-351A (a radiolabeled ACE inhibitor) binding. The studies indicate that the tissue-sampling method significantly underestimated functional ACE inhibition in hamster papillary muscles (p < 0.001). Kinetic studies indicated that the half-time for the dissociation of [3H]enalaprilat and [3H]lisinopril from hamster ventricular ACE was 4.5 and 6.2 minutes, respectively. The dissociation of [3H]quinaprilat was biphasic (half-time, 47 and 90 minutes), indicating that the two active sites of somatic ACE differ in their ability to bind to this inhibitor. The rapid rate of ACE inhibitor dissociation suggests that, during the time taken to assay ACE activity biochemically, the enzyme becomes "disinhibited," leading to an underestimation of functional ACE inhibition. ACE inhibitor dissociation rates were partially predictive of the duration of functional ACE inhibition in papillary muscles; other factors that appeared to contribute were "tissue trapping" of the inhibitor and de novo synthesis of ACE in papillary muscles. Quantification of tissue ACE inhibition and its relation to drug efficacy must, therefore, involve a careful consideration of these factors to avoid artifacts in clinical decision making and in assessments of pathogenic mechanisms involved in congestive heart failure.