At present it is not clear if male fertility is affected by intermittent hypobaric hypoxia (IHH).This is an important issue since a large human population works over 3000 masl.This study analyzes epididymal sperm, in adult Sprague Dawley rats after five cycles of IHH (7 days exposure to 4200 masl in a hypobaric chamber / 7 days at 500 masl).The animals were separated into groups of 8, one group was exposed to hypoxia (7 days), and the others to IHH for one to five cycles.Controls (500 masl) were examined at the beginning and at the end of the 70 experimental days.A duplicate set of rats treated with melatonin (supposedly protecting from hypoxia) was also examined, as were their controls, injected with 0,03% ethanol (melatonin solvent).Epididymal sperm parameters, were evaluated.Damage caused by IHH increases with time.Sperm counts drop, while sperm chromatin swelling, DNA instability (metachromasia with acridine orange epifluorescence) and comet (+) tests increase.Melatonin counteracts all this damage, possibly due to its high efficiency as a reactive oxygen species scavenger.In conclusion, IHH exposure damages sperm quality and therefore male reproductive function.