To examine the effects of uncontrollable and controllable electric shocks on the immune system in rats, the proportion of CD4+, CD8+ or CD25+ T lymphocytes to total lymphocytes was measured in the peripheral blood, spleen, and thymus. The rats were given either controllable shocks, identical uncontrollable shocks, or no shocks, and then small shocks 24 h later (reinstating shocks). The proportion of CD4+ T lymphocytes relative to total lymphocytes in both the peripheral blood and spleen of uncontrollable rats (URs) was significantly smaller than was found in no-shock rats (NRs). Similarly, the proportion of CD4+ T lymphocytes in the thymus of controllable rats (CRs) was significantly smaller than in NRs. In contrast, the proportion of CD8+ T lymphocytes in the thymus of URs was significantly larger than in NRs and CRs. The CD4+ to CD8+ T lymphocyte ratios (CD4+/CD8+ ratios) in the peripheral blood, spleen and thymus of URs were significantly smaller than in NRs; also, the ratios in the peripheral blood and spleen of URs were significantly smaller than in CRs. The white blood cell (WBC) count of URs was significantly smaller than those of NRs and CRs, and the WBC count of CRs was significantly smaller than those of NRs and CRs, and the WBC count of CRs was significantly smaller than that of NRs. These results suggest that decreases in CD4+ T lymphocytes (and/or an increase of CD8+ T lymphocytes) in the peripheral blood, spleen, and thymus are caused by uncontrollable stress followed by a reinstating stress condition, leading to the decrease of WBC in the peripheral blood and decreases in the CD4+/CD8+ ratios in these tissues.