Previously, we reported regional variations in the number and subsets of the small intestinal IELs of mice. In this study, we examined the age-related changes in the regional variations of IELs in mice from 2 to 11 weeks old. IELs were isolated from the proximal, middle and distal parts of the small intestine and analysed by flow cytometry. The total number of IELs gradually increased with age and reached a plateau at 8 weeks old. As to IEL subsets, the percentage of alpha beta T cells was higher in the distal part at and after 2 weeks of age (before weaning). The percentage of the alpha beta T cell subset of extrathymic origin was higher in the proximal part while the percentages of alpha beta T cell subsets of thymic origin were higher in the distal part at and after 3 weeks (just after weaning). It appears that regional variations in IELs may be formed before the weaning period in mice.