Sustained weightlessness affects all body functions, among these also cardiac autonomic control mechanisms. How this may influence neural response to central stimulation by a mental arithmetic task remains an open question. The hypothesis was tested that microgravity alters cardiovascular neural response to standardized cognitive load stimuli. Beat-to-beat heart rate, brachial blood pressure and respiratory frequency were collected in 5 astronauts, taking part in 3 different short duration (10-11 days) space missions to the International Space Station. Data recording was performed in supine position one month before launch, at day 5 or 8 in space, and on day 1, 14 and 25 days after landing. Heart rate variability parameters were obtained in the frequency domain. Measurements were performed in the control condition for 10 minutes and during a 5 minute mental arithmetic stress task, consisting of deducting 17 from a 4 digit number, read by a colleague and orally announce the result. Our results show that over all sessions (pre-, in- and post-flight), mental stress induced an average increase in mean heart rate (7 ± 1 beats/min, p=0.03) and mean arterial pressure (7 ± 1 mmHg, p=0.006). A sympathetic excitation during mental stress was shown from HRV parameters: increase of LFnu (8.3 ± 1.4, p = 0.004) and LF/HF (1.6 ± 0.3, p = 0.001) and decrease of HFnu (8.9 ± 1.4, p = 0.004). The total power was not influenced by mental stress. No effect of space flight was found on baseline heart rate, mean arterial pressure and HRV parameters. No differences in response to mental stress were found between pre-, in- and post-flight. Our findings confirm that a mental arithmetic task in astronauts elicits sympathovagal shifts towards enhanced sympathetic modulation and reduced vagal modulation. However, these responses are not changed in space during microgravity or after space flight.