Serelaxin, recombinant human relaxin-2, modulates endothelial vasodilatory functionality and is under evaluation for treatment of acute heart failure. Little is known about acute effects on cerebral perfusion. We tested the hypothesis that Serelaxin might also have effects on the cerebral microcirculation in a sheep model, which resembles human brain structure quite well. We used laser Doppler flowmetry and sidestream dark-field (SDF) imaging techniques, which are reliable tools to continuously assess dynamic changes in cerebral perfusion. Laser Doppler flowmetry shows that bolus injection of 30 g Serelaxin/kg body wt induces an increase (P ϭ 0.006) to roughly 150% of cortical cerebral blood flow (CBF), whereas subcortical CBF remains unchanged (P ϭ 0.688). The effects on area-dependent CBF were significantly different after the bolus injection (P ϭ 0.042). Effects on cortical CBF were further confirmed by SDF imaging. The bolus injection of Serelaxin increased total vessel density to 127% (P ϭ 0.00046), perfused vessel density to 145% (P ϭ 0.024), and perfused capillary density to 153% (P ϭ 0.024). Western blotting confirmed the expression of relaxin receptors RXFP1 and truncated RXFP2-variants in the respective brain regions, suggesting a possible contribution of RXFP1 on the effects of Serelaxin. In conclusion, the injection of a high dose of Serelaxin exerts quick effects on the cerebral microcirculation. Therefore, Serelaxin might be suitable to improve cortical microcirculation and exert neuroprotective effects in clinically relevant scenarios that involve cortical hypoperfusion. These findings need to be confirmed in relevant experimental settings involving cerebral cortical hypoperfusion and can possibly be translated into clinical practice. RELAXIN-2 IS A HUMAN HORMONE that mediates cardiovascular adaptations during pregnancy and particularly systemic and renal vasodilatation (7). Moreover, these effects are observed in nonpregnant animals and in ex vivo studies following the artificial administration of the substance as well (33). An intravenous bolus injection of a recombinant form of human relaxin-2, Serelaxin (Novartis Pharma, Basel, Switzerland), modulates a sustained endothelial vasodilatory function. Serelaxin mediates systemic hemodynamic changes and increases renal blood flow through a fall in systemic intravascular resistance with a concomitant increase in arterial compliance (15,22,31,38,39). In the RELAX-AHF study, cardiovascular and all-cause mortality was reduced 37% in patients treated with Serelaxin (42). Furthermore, initial positive studies on the cerebral effects of this peptide hormone in a stroke model (47) and in a high blood pressure situation (6) have already been published. Serelaxin induces a nitric oxide-mediated vasodilatation (40) and can particularly reduce the infarction size of the cerebral cortex, suggesting a neuroprotective effect (47).Nonetheless, studies on the influence of Serelaxin on the cerebral blood flow (CBF) have not been performed to this day. Moreover, desp...