During their stay in a space habitat, astronauts are exposed to many different stressors that may entail detrimental effects on mood and performance. In order to monitor the effects of the space environment on different human information processing functions during an extraordinary long-term space mission, the cognitive. visuo-motor and time-sharing performance of one Russian cosmonaut was repeatedly assessed (29 times) during his 438-day stay in space. The performance tasks used were chosen from the AGARD-STRES battery and included grammatical reasoning, Sternberg memory-search, unstable tracking, and a dual-tasks consisting of unstable tracking with concurrent memory-search. In addition to performance assessment, several subjective ratings concerning mood and workload were collected. Comparisons of pre-flight, in-flight, post-flight and two follow-up assessments 6 months after the mission revealed, (1) no impairments of basic cognitive functions during the flight, (2) clear impairments of mood, feelings of raised workload, and disturbances of tracking performance and time-sharing during the first 3 weeks in space and the first 2 weeks after return to Earth, (3) an impressive stability of mood and performance during the second to fourteenth month in space, where mood and performance had returned to pre-flight baseline level, and (4) no long-lasting performance deficits at follow-up assessments. From these results it is concluded that the first 3 weeks of long-term spaceflights and the first 2 weeks back on Earth represent critical periods where adverse effects on attentional processes are to be expected, induced by the demands to adjust to the extreme environmental changes. The stability of mood and performance observed after successful adaptation to the space environment indicates that mental efficiency and emotional state can be maintained on a level as high as on Earth even during extraordinary long-term space missions.