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Determination of reproductive season is based on the presence of pregnant females and male testis position and size in several micro-mammals in wild life. However, the endocrine function of the testicles is unknown. Our goal was to evaluate, on a monthly basis during the two reproductive seasons of the year, the relationship between circulating testosterone level and testis recrudescence on adult males of free living Black-eared deermouse Peromyscus melanotis from a mid-latitude temperate forest. Our evidence shows a pattern of raise and fall, though decoupled by a month, between both processes, which sheds light on their physiological relationship in this species, as well as about its plasticity to respond to environmental conditions. They also warn us against customarily inferences about reproductive activity based only in position and size of testes. Finally, our evidence contributes to the scarce knowledge of the endocrine function in some particular events of the reproductive biology of Peromyscus melanotis, and on the reproductive biology of Peromyscus in temperate mid-latitudes.