Serelaxin is a novel recombinant human relaxin-2 that has been investigated for the treatment of acute heart failure. However, its effects on renal function, especially on the renal microcirculation remain incompletely characterized. Our immunoexpression studies localized RXFP1 receptors on vascular smooth muscle cells and endothelial cells of afferent arterioles and on principal cells of collecting ducts. Clearance experiments were performed in male and female normotensive rats and Ang II infused male rats. Serelaxin increased mean arterial pressure slightly and significantly increased renal blood flow, urine flow and sodium excretion rate. Group analysis of all serelaxin infusion experiments showed significant increases in GFR. During infusion with subthreshold levels of Ang II, serelaxin did not alter mean arterial pressure, renal blood flow, GFR, urine flow or sodium excretion rate. Heart rates were elevated during serelaxin infusion alone (37±5%) and in Ang II infused rats (14±2%). In studies using the in vitro isolated juxtamedullary nephron preparation, superfusion with serelaxin alone (40 ng/ml) significantly dilated afferent arterioles (10.8±1.2 vs. 13.5±1.1 µm) and efferent arterioles (9.9±0.9 vs. 11.9±1.0 µm). During Ang II superfusion, serelaxin did not alter afferent or efferent arteriolar diameters. During NO synthase inhibition (L-NNA), afferent arterioles also did not show any vasodilation during serelaxin infusion. In conclusion, serelaxin increased overall renal blood flow, urine flow, GFR and sodium excretion and dilated the afferent and efferent arterioles in control conditions, but these effects were attenuated or prevented in the presence of exogenous Ang II and NO synthase inhibitor.