Corticotrope or ACTH cells were immunohistochemically identified in the pituitary pars distalis (PD) of viscacha by using the polyclonal antiserum against ACTH (1-24). The localization, distribution, shape, percentage immunopositive area, and major cellular and nuclear diameters of these cells were analyzed by image analysis in adult male viscachas captured in their natural habitat during the year and after the chronic administration of melatonin. The same parameters were analyzed in immature male and adult female viscachas. ACTH cells in adult males were mainly localized in the dorsal and cephalic regions of PD. They were isolated, forming small groups and in contact with follicular structures and blood vessels. They were pleomorphic, with some of them being polygonal, oval, round, and others, stellate with cytoplasmic extensions. The percentage immunopositive area and the major cellular diameter showed seasonal variations with lower values during June and July (early winter). A decrease in the percentage immunopositive area was observed after the administration of melatonin in adult male animals. ACTH cells of immature animals differed from the adults' cells in their distribution, shape, pattern of immunolabeling, and percentage immunopositive area. These parameters in adult females did not vary in relation to adult males at the same time of the year although. However, the cells in females were smaller in size during April-May. In pregnant viscachas (June-July), these parameters did not show significant differences with the results of non-pregnant females (April-May). This suggests that the environmental stressors do not exert the same influence on the hypothalamo-pituitary-adrenal axis in adult male and female viscachas, probably due to the physiological changes caused by pregnancy. Our results in adult male viscacha demonstrate that the morphology of the ACTH cells varies according to the different seasonal conditions, thus participating in the adaptation process of this rodent to the environment. The elevated levels of melatonin during winter months might inhibit the synthesis of ACTH, probably when affecting some secretagogue of this pituitary hormone. Moreover, the morphological variations observed between adult and immature male viscachas and between both sexes suggest that the steroid gonadal hormones might act on the development and activity of ACTH cells.