Abdominal aortic pressure (AAP), heart rate (HR), and aortic nerve activity (ANA) during parabolic flight were measured by using a telemetry system to clarify the acute effect of microgravity (microG) on hemodynamics in rats. While the animals were conscious, AAP increased up to 119 +/- 3 mmHg on exposure to microG compared with the value at 1 G (95 +/- 3 mmHg; P < 0.001), whereas AAP decreased immediately on exposure to microG under urethane anesthesia (microG: 72 +/- 9 mmHg vs. 1 G: 78 +/- 8 mmHg; P < 0.05). HR also increased during microG in conscious animals (microG: 349 +/- 12 beats/min vs. 1 G: 324+9 beats/min; P < 0.01), although no change was observed under anesthesia. ANA, which was measured under anesthesia, decreased in response to acute microG exposure (microG: 33 +/- 7 counts/s vs. 1 G: 49 +/- 5 counts/s; P < 0.01). These results suggest that microG essentially induces a decrease of arterial pressure; however, emotional stress and body movements affect the responses of arterial pressure and HR during exposure to acute microG.