The depressor and renal responses to C-type natriuretic peptide (CNP) were determined in conscious cynomolgus monkeys treated with vehicle or inhibitors of neutral endopeptidase EC 3.4. 24.11 (NEP) and angiotensin-converting enzyme (ACE). The NEP inhibitor SQ 28603 (100 μmol/kg intravenously, iv) significantly (p< 0.05) enhanced the depressor responses to 1 and 10 nmol/kg iv CNP from-2±3 to-22±10 mm Hg and from-16±4 to-66±4 mm Hg, respectively. SQ 28603 also significantly increased the cyclic GMP responses to 1 and 10 nmol/kg CNP from 1.4±1.6 to 11.0±2.0 nmol/2 h and from 4.2±0.5 to 53.3±12.1 nmol/2 h, respectively. Furthermore, the NEP inhibitor significantly increased the natriuretic activity of 1 and 10 nmol/kg iv CNP from 235±99 to 760±60 μEq/2 h and from 399±208 to 1,036±79 μEq/2 h, respectively. A positive correlation between the cumulative natriuretic and cyclic GMP responses suggested a cyclic GMP-mediated mechanism. These data are consistent with the protection of CNP from degradation by renal NEP. Inhibition of ACE by 100 μmol/kg iv captopril did not significantly alter the depressor or renal activities of 1 nmol/kg of CNP, neither did it alter the potentiation of CNP activity by SQ 28603. The potentiation of the depressor, cyclic GMP, and natriuretic responses to CNP in nonhuman primates by SQ 28603 suggested that NEP is an important mechanism for in vivo inactivation of natriuretic peptides, including CNP.