The present study was performed to elucidate the effects of physical fatigue on immune parameters of peripheral blood and splenic T cells in rats. Male Wistar rats aged 20 to 45 weeks were used for the experiment. Swimming was employed as a physical fatigue load. The rats in the single-load group with a weight equal to 3% of the body weight of each rat connected to the tails were exhausted by two hours of swimming in one day. The rats in the consecutive-load group were exhausted by three hours of swimming for 7 consecutive days with a weight equal to 5% of the body weight of each rat. Absolute numbers of total white blood cells (WBCs) and peripheral blood lymphocytes decreased significantly just after swimming and returned to the baseline one day after swimming in both the single- and consecutive-load groups. However, in the consecutive-load group WBCs and lymphocytes decreased 7 days after swimming. Concentrations of plasma corticosterone increased significantly just after swimming, and returned to the baseline one day after swimming in both groups. Percentages of peripheral blood CD5+ cells, CD4+ cells and CD8+ cells in the single-load group increased significantly just after swimming. The percentage of peripheral blood CD5+ cells and CD4+ cells increased significantly one day after swimming in the consecutive load group. Percentages of splenic CD5+ cells and CD4+ cells increased significantly in the single-load group. However, only that of CD4+ cells increased significantly just after swimming in the consecutive-load group. A single load affected the proportions of CD5+ cells, CD4+ cells and CD8+ cells just after the load but the consecutive load affected CD5+ cells and CD4+ cells and the effect persisted until the next day. These results indicate that the effect of fatigue on immune parameters depends on the level of fatigue, and that measurement of subsets of peripheral blood T cells is useful for evaluating various levels of fatigue.