The influence of neutral endopeptidase (NEP) inhibition with (S)-thiorphan on the hormonal, renal, and blood-pressure-lowering effects of an infusion of atrial (ANP), brain (BNP), and C-type natriuretic peptide (CNP) was evaluated in hypertensive transgenic rats (TGR) harboring an additional mouse renin gene (TGR(m(Ren2)27)). These TGR possess an activated natriuretic peptide system as compared with Sprague-Dawley rats (SDR), used in this study as control. (S)-Thiorphan significantly decreased blood pressure in anesthetized TGR but not in anesthetized SDR during the 60-min infusion period. Exogenously administered ANP decreased blood pressure in SDR with no significant effects in TGR after 60 min. In contrast, BNP infusion significantly decreased blood pressure in TGR, while changes in SDR were not significant. The blood pressure was further decreased after combined infusion of ANP and BNP with (S)-thiorphan in TGR. No effect on blood pressure was registered during infusion of CNP in either experimental group. The plasma levels of ANP, BNP, and cGMP were higher in TGR than in SDR, whereas plasma renin activity was lower. Co-administration of ANP, BNP, or CNP with the NEP inhibitor (S)-thiorphan potentiated the plasma ANP, BNP, and cGMP. Infusion of ANP alone did not affect BNP plasma levels of TGR and vice versa. In contrast, CNP infusion increased ANP plasma levels in both TGR and SDR. Renal excretion of sodium and cGMP increased after infusion of (S)-thiorphan and ANP or BNP in both TGR and SDR. The combination of ANP and (S)-thiorphan had a slightly greater effect on urinary excretion of sodium and cGMP in TGR than either compound alone, but the effects were more pronounced in SDR than in TGR. Finally, infusion of CNP alone and in combination with (S)-thiorphan influenced the excretion of sodium and cyclic GMP only slightly. These results indicate that inhibition of neutral endopeptidase by (S)-thiorphan potentiates the hemodynamic and renal effects of natriuretic peptides ANP and BNP, and to some extent those of CNP, in hypertensive TGR and normotensive SDR. In contrast to ANP and BNP, infusion of CNP had no effect on the blood pressure in anesthetized TGR or SDR. Inhibition of NEP therefore seems to be a promising way to potentiate endogenous levels of natriuretic peptides, which may be of therapeutic benefit in cardiovascular diseases such as hypertension or heart failure.