Whether myocardial contractile impairment contributes to orthostatic intolerance (OI) remains controversial. Accordingly, we have performed a three part study in which we have compared the in-vivo pressor, chronotropic, and inotropic responses (Part 1 and Part 2) to an open loop selective carotid baroreceptor unloading by performing transient bilateral carotid occlusion (TBCO) in anesthetized mice, and (Part 3) in vitro myocyte responses to isoproterenol between mice exposed to hind-limb unweighting (HLU) for ~2 weeks and controls. In part 1, we developed a common protocol in which we bilaterally exposed the carotid sinus regions to separate carotid artery, vagus, aortic depressor nerves (ADN) and sympathetic trunk (ST). We measured HR and MAP responses to TBCO in several groups of mice. In controls (C), TBCO increased heart rate (HR; 15±2 bpm, P<0.05) and mean arterial pressure (MAP; 17±2 mmHg, P<0.05). These responses were markedly potentiated in denervated controls (DC) in which the aortic depressor nerve (ADN) and sympathetic trunk (ST) were sectioned before measurement. Baroreflex responses of TBCO were eliminated by ganglionic blockade (hexamethonium bromide, 10 µg/kg). Like DC, HLU mice were also denervated prior to TBCO, and exhibited HR and MAP responses that were ~70% reduced compared to DC. In Part 2 of this study, myocardial contractile responses to TBCO were measured with a LV micromanometer-conductance catheter in additional groups of DC and HLU mice. TBCO in DC increased the slope of the end-systolic pressure volume relation (Ees) by 86±13%. This inotropic response was markedly attenuated (14±10% (P<0.005)) after HLU. Furthermore, in Part 3, myocytes isolated from HLU had impaired contractile responses to isoproterenol. In conclusion, selective carotid baroreceptor unloading stimulates HR, blood pressure, and myocardial contractility, and HLU attenuates each of these responses. These findings have important implications for the management of orthostatic intolerance (OI) in astronauts, the elderly, and individuals exposed to prolonged bedrest.