The mammary gland has long drawn the attention of the scientific community due to the limited knowledge of some fundamental aspects involved in the control of its function. Myotis velifer, a microchiropteran species, provides an interesting model to study some of the regulatory factors involved in the control of the mammary gland cycle. Having an asynchronous, monoestrous reproductive pattern, female M. velifer bats undergo drastic morphological changes of the breast during the reproductive cycle. Current research on non-chiropteran mammals indicates that serotonin (5-HT) plays a major role in the intraluminal volume homeostasis of the mammary gland during lactation; however, an analysis of both the expression and localization of the main components of the serotonergic system in the bat mammary gland is lacking. Thus, the objectives of the present study were: to describe the gross and histological anatomy of the mammary gland of M. velifer to establish the lactation period for this species; to analyze the distribution and expression of the main serotonergic components in the mammary tissues of these bats under the physiological conditions of lactation, involution and the resting phase; and to provide information on the involvement of 5-HT in the regulation of the physiological function of this organ. To assess the expression and localization of serotonergic components, multiple immunofluorescence, Western blot and HPLC methods were used. 5-HT and the enzyme that catalyzes its synthesis (TPH) were located in both myoepithelial and luminal epithelial cells, while the enzyme responsible for the catabolism of this neurohormone (MAO A) was found in luminal epithelial cells as well as in secreted products. We also found an increased expression of serotonergic components during lactation, indicating that elements of the serotonergic system may play an important role in lactation in this species of bat in a way similar to that of other mammal species.