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The potent neutral endopeptidase inhibitor SQ 28 603 (N-(2-(mercaptomethyl)-1-oxo-3-phenylpropyl)-β-alanine) significantly increased excretion of sodium from 4.9 ± 2.3 to 14.3 ± 2.1 μequiv./min and cyclic 3′,5′-guanosine monophosphate from 118 ± 13 to 179 ± 18 pmol/min after intravenous administration of 300 μmol/kg (~80 mg/kg) in conscious female cynomolgus monkeys. SQ 28 603 did not change blood pressure or plasma atrial natriuretic peptide concentrations in the normal monkeys. In contrast, 1-h infusions of 3, 10, or 30 pmol∙kg−1∙min−1 of human atrial natriuretic peptide lowered blood pressure by −3 ± 4, −9 ± 4, and −27 ± 3 mmHg (1 mmHg = 133.322 Pa), increased cyclic guanosine monophosphate excretion from 78 ± 11 to 90 ± 6, 216 ± 33, and 531 ± 41 pmol/min, and raised plasma atrial natriuretic peptide from 7.2 ± 0.7 to 21 ± 4, 62 ± 12, and 192 ± 35 fmol/mL without affecting sodium excretion. In monkeys receiving 10 pmol∙kg−1∙min−1 of atrial natriuretic peptide, 300 μmol/kg of SQ 28 603 reduced mean arterial pressure by −13 ± 5 mmHg and increased sodium excretion from 6.6 ± 3.2 to 31.3 ± 6.0 μequiv./min, cyclic guanosine monophosphate excretion from 342 ± 68 to 1144 ± 418 pmol/min, and plasma atrial natriuretic peptide from 124 ± 8 to 262 ± 52 fmol/mL. In conclusion, SQ 28 603 stimulated renal excretory function in conscious monkeys, presumably by preventing the degradation of atrial natriuretic peptide by neutral endopeptidase.Key words: atrial natriuretic peptide, neutral endopeptidase, natriuresis, cyclic guanosine monophosphate.