Increase of spacefl ight duration, long-term hermetic capacity projects together with challenges of health status maintenance and performance capability of test researchers raise a question about need for research of reproductive function under that condition. Data on eff ect of long isolation condition on male reproductive system status is extremely limited. Th e aim of present study was examination of functional status of male reproductive system during 105-day dwelling under condition of earth hermetic object as a preliminary step of program of Mars mission preparations (program "MARS-500"). Six substantially healthy men volunteers 26 – 41 years old were selected. Aft er 105 days evaluation of male sexual function was performed using BSF-I criteria. Baseline indices were within WHO standards. Long isolation in hermetic object along with signifi cant hypokinesia resulted in substantial alteration of main spermatogenesis characteristics in test subjects. Aft er 105 days dwelling under isolation conditions negative infl uence thereof was detected in ejaculate analyses which manifested in substantial increase of pathological forms of sperm cells head by 30%. At the same time signifi cant ejaculate volume expansion, increase of sperm cells concentration and certain increase of active sperm motility were registered. Th ere was no signifi cant change in sex hormones level. Th e fi ndings may have practical importance in examination of cohorts that are in equal isolation conditions including following spacefl ight programs.