In this study, we test the hypothesis that body size is a key factor in determining dominance relationships during agonistic encounters between captive males Caluromys philander, and male-male competition was experimentally tested. Physiological and behavioural parameters were investigated during 18 experimental trials including two males alone (6), and two males with a female (12). A dominance-submission relationship exists even with no female, although competition is stronger when a female is present. Dominance is based on age and body mass in C. philander under experimental conditions. When dominance was settled, dominance rank was clear cut and consistent during all the experimentation. Stable dominance-subordination relationships may benefit both dominants and subordinates by minimizing the incidence of serious wounds. Males show typical signs of social stress : both body weight and hematocrit rate decrease, but subordinates are more stressed than dominants. Dominant males exhibit a lower decrease in hematocrit rate and a higher decrease in testosterone concentration in blood. They display most of the “aggressive” behaviours, scan more actively the experimental cages and engage in fewer aggressive interactions with females in comparison with subordinate males.