To what extent does going to space affect cardiovascular function? Although many studies have addressed this question, the answer remains controversial. Even for such primary parameters as heart rate (HR) and blood pressure (BP) contradictory results have been presented. The purpose of this investigation was to evaluate HR and arterial BP in 11 male astronauts who each took part in nine different space missions aboard the International Space Station (ISS), for up to 6 months. Pre-flight HR and BP readings were obtained in both the standing and supine positions on Earth and were taken as reference values. Our results show that HR and arterial BP in space equal pre-flight supine values. In all subjects, HR and mean arterial BP (MAP) were lower in space compared with pre-flight standing (both p<0.05). HR in space was well maintained at pre-flight supine level for up to 6 months in all astronauts while MAP tended to adapt to a level in between the ground-based standing and supine positions. Also pulse pressure (PP) decreased over the course of long duration spaceflight. In conclusion, our data indicate that weightlessness relaxes the circulation in humans for an extended duration of up to 6 months in space.