A comparative study was made for evaluation of peripheral blood T lymphocyte subsets in child (40 days) and adult (5-6 months)rats.There was significant difference between child and adult rats in peripheral blood T cell subsets: in the child, there were more OX-l9+(pan T cell)and W3/25+(Th/i cell) cells, and less OX-8+ (Ts/c cell)cells as compared to the adult. In W3/25+ and OX-8+ subsets, however, the fluorescence intensity of cell surface in FCM histogram presented oppositc trend in comparison with the percentage of these cells. It is assumed that the reason resulting in such change is either T cell maturation of the down regulation of T cell antigen receptors.The data described here are provided with some important information for further study in rat T cell maturation.