High levels of cooperation and low aggression seem obviously vital to the successful implementation of space missions. To elucidate the effect of microgravity on these behaviors, we investigated whether cooperative and aggressive behaviors would be affected in 16 male volunteers during 45-day -6 degrees head-down bed rest, which is a reliable simulation model for most physiological effects of spaceflight. We used an ultimatum game task to evaluate the cooperative behavior and a revised competitive reaction time test to evaluate the aggressive behavior simultaneously. We found that (1) the participants became less cooperative in the post-bed rest phase in comparison with the pre-bed rest phase and (2) the participants became more aggressive in the in-bed rest phase in comparison with the pre-bed rest phase. These findings provide evidence that head-down bed rest may affect both cooperative and aggressive behaviors in males, suggesting an important perspective for future studies in space psychology.Copyright (c) 2017 John Wiley & Sons, Ltd.