Aging is associated with changes in T cells including involution of the thymus gland and an imbalance in the proportion of näive (CD44lo) and memory (CD44hi) T cells in the periphery. Reversal of these changes may improve immunity in the aged. We sought to determine whether 4 months of moderately intense treadmill running (EXC; 5 days/week, 45 min/day, 13-22 m/min) in 2 month (Y) and 18 month (O) old male Balb/c mice would alter T lymphocyte profiles in the thymus and spleen when compared to sedentary controls (CON). Splenocytes and thymocytes were harvested 24-48 h after the last exercise session and analyzed using immunofluorescence and flow cytometry. While there were significant age-related changes (lower cell number, altered subsets) in the thymuses of O when compared to Y mice, exercise training failed to affect any of these measures in mice of either age. Aged mice exhibited a significantly (p < .05) higher percentage of splenic memory cells and a lower percentage of näive cells in both the CD4 and CD8 T cell subsets. Interestingly, exercise training significantly (p < .05) increased the percentage of näive and decreased the percentage of memory cells in both the CD4+ (69.6+/-1.7% näive and 30.4+/-1.7% memory for OCON vs. 75.0+/-1.5% näive and 25.0+/-1.5% memory in OEXC) and CD8+ (60.0+/-2.6% näive and 40.0+/-2.6% memory in OCON vs. 76.7+/-2.7% näive and 23.3+/-2.7% memory in OEXC) T cells subsets in O, but not Y, mice. This effect was due to a decrease in the absolute number of memory cells and not an increase in the absolute number of näive cells. We conclude that 4 months of EXC has little restorative effect on the thymus in aged mice, but can restore the percentages of näive and memory cells in the spleen towards that of young mice, perhaps due to removal of memory cells.