Abstract--Chronic administration of recombinant human relaxin (rhRLX) to conscious, normotensive rats (male and female) increases cardiac output (CO) and global arterial compliance (AC_g) and reduces systemic vascular resistance (SVR) with no change in mean arterial pressure (MAP). Effects (magnitude and temporal pattern) of relaxin on systemic hemodynamics and arterial properties in hypertensive animal models are not known. Accordingly, the major goal of the present study was to determine the cardiovascular effects of rhRLX in hypertensive rats using 2 models: Long-Evans rats chronically administered angiotensin II (AII) and spontaneously hypertensive rats (SHR). CO and systemic arterial load, as quantified by SVR and AC_g, were obtained using methods reported previously by us. In rats with AII-induced hypertension, acute rhRLX administration (up to 6 hours) significantly increased CO and AC_g (24.9±3.9 and 34.3±12.6% above baseline, respectively) and significantly decreased SVR (17.2±3.5%) without changing MAP. In contrast, acute rhRLX administration to SHR and normotensive rats for up to 6 hours failed to produce any significant changes in CO, AC_g, SVR, or MAP. However, chronic rhRLX administration (1 to 7 days) to SHR yielded significant changes (24.0±8.1 and 22.3±6.6% increases in CO and AC_g, respectively, and a 13.3±5.3% decrease in SVR, with no change in MAP). In conclusion, rhRLX increases CO and reduces arterial load in hypertensive rats without reducing MAP. However, the time course of response to rhRLX treatment is dependent on the model of hypertension such that rats characterized by AII-mediated hypertension responded more rapidly to rhRLX administration than SHR.