Reproduction implies high energetic cost to animals; therefore it should be synchronized with the most favorable season of the year. Photoperiod could act as a cue for regulation of reproduction in mammals from seasonal habitats. Thylamys pallidior is a marsupial inhabiting the Monte desert of Argentina; it reproduces once in its life time, synchronizing with high food resource seasons. However, only those individuals able to cope with winter conditions will reproduce. Our objective was to investigate the role of photoperiod as a potential cue for the onset of reproduction and its effect on metabolism as an indicator of energetic performance. Male T. pallidior were captured in the Monte Desert; five were maintained under natural photoperiod (NP) and five under constant LD 12:12 and 10:14 (winter; CP) for 12 months. We recorded body weight, tail and scrotal width, testes pigmentation and spermathorroea. Metabolic rate and changes in body temperature were measured for individuals from each treatment at different temperatures (5-38°C) . We found no difference in scrotal-related variables between treatments, individuals from NP showed spermathorroea 2 months earlier than CP. We found different patterns of tail width variation between treatments. No statistical differences were found for metabolic rates and body weight loss. However, NP animals showed decreases in body temperature and metabolic rate at the end of trials compared to CP. These tendencies seem to indicate some effect of photoperiod in regulating reproduction and in performance of animals under low temperatures. Further studies are necessary to explore and confirm these results.