Adrenocortical X-zone, a transitory cortical region in virgin female and immature male, exists exclusively in mice, and its detailed nature, regression and turnover remain unclear. In this study, the X-zone was investigated in pregnant Jcl:ICR mice, in aspects of morphological traits, cell proliferation by bromodeoxyuridine (BrdU) immunohistochemistry and apoptosis by in situ nick endlabeling of genomic DNA (TUNEL). In the early pregnancy, the intact X-zone, which took more than 30 % of the total adrenocortex, was composed exclusively of non-vacuolated cells, with smaller size and a darker nuclei than other permanent cortical cells. From day 8 of gestation, the X-zone involuted gradually and this degeneration process lasted about one week. There are similarities and discrepancies between Jcl:ICR mice and reported other strains in cellular composition and period of the involution, suggesting strain-difference due to genetic background. BrdU immunohistochemistry demonstrated an apparently more active cell proliferation in the intact X-zone than in other permanent cortical zones. Apoptotic cells were not observed during the X-zone involution by either hematoxylin-eosin staining or TUNEL.