Pancreatic islets adapt to metabolic requirements and the hormonal milieu by modifying their size and hormone secretions. Maternal glucose demands and hormonal changes occur after weaning, to rapidly re-establish bone mineralization. Minimal information exists about glucose metabolism and pancreatic islets after lactation. This study investigated islet morphology and glucose homeostasis for 14 days after lactation in C57BL/6NHHsd mice. Compared to the day of weaning, rapid increases in the islets’ area and number of beta cells were found from the first day post lactation, attaining maximum values on the third day post weaning. These changes were accompanied by modifications in glucose-induced insulin secretion, glucose tolerance and insulin sensitivity. Islet-cell proliferation was already augmented before lactation ceased. Serum undercarboxylated osteocalcin concentrations increased significantly post lactation; however, it is unlikely that this enhancement participates in earlier cell proliferation augmentation or in decreasing insulin sensitivity. Islet serotonin content was barely expressed, and serum calcium concentrations decreased. By the 14th day post weaning, islets’ area and glucose homeostasis returned to age-matched virgin mice levels. These findings recognize for the first time that increases in islet area and insulin secretion occur during physiological post-weaning conditions. These results open up new opportunities to identify molecules and mechanisms participating in these processes, which will help in developing strategies to combat diabetes.