Extensive evidence has revealed variations in the number of hormone-producing cells in the pituitary gland, which occur under physiological conditions such as gestation and lactancy. It has been proposed that new hormone producing cells differentiate from stem cells. However, exactly how and when this takes place is not clear. In this work, we used immunoelectron-microscopy to identify adult pituitary stem/progenitor cells (SC/P) localized in the marginal zone (MZ) and additionally, we detected GFRa2, Sox2 and Sox9 positive cells in the adenoparenchyma (AP) by fluorescence microscopy. Then, we evaluated fluctuations of SC/P mRNA and protein level markers in MZ and AP during gestation and lactancy. An up-regulation in stemness markers was shown at term of gestation (AT) in MZ, while, there was more progenitor cell marker at middle of gestation and active lactancy. Concerning committed cell marker, we detected a rise in AP at beginning of lactancy (d1L). We performed a BrdU-uptake analysis in MZ and AP cells. The highest level of BrdU-uptake was observed in MZ AT cells whereas, in AP, this was detected in d1L, followed by a decrease in both the MZ and AP. Finally, we detected double-immunostaining for BrdU-GFRa2 in MZ AT cells and BrdU-Sox9 in the AP d1L cells. Taken together, we hypothesize that the expansion of the SC/P niche took place mainly in MZ from pituitary rats in AT and d1L. These results suggest that the SC niche actively participates in pituitary plasticity during these reproductive states, contributing to the origin of hormone cell populations.