Corticosterone (CORT) belongs to a family of steroid hormones known as the glucocorticoids. CORT is produced in vivo in response to physical and psychological stress and has immunosuppressive qualities. As an immunosuppressant, CORT alters the frequency of immune cells in lymphoid organs. This study quantified the effects of exogenous CORT on splenic CD8 T cells in mice. CD8 T cells are critical for destruction of virally infected and tumorigenic host cells. Thus, it is important to determine the timing by which CORT alters the frequency of CD8 T cells. CORT was administered in the drinking water of C57BL/6 mice for time intervals ranging from 12-120 hours. Following treatment, spleens were harvested and processed, total splenocytes were counted, and CD8 T cells were quantified by flow cytometry. Spleens from mice treated with CORT were significantly smaller than spleens from untreated mice following just 12 hours of exposure to CORT. A marked decline in both naïve (CD44lo) and activated (CD44hi) CD8 T cells was observed. This loss of CD8 T cells was greater the longer the mice were treated with CORT. Flow cytometric analysis suggests that this decrease in CD8 T cells was due, in part, to increased amounts of apoptosis in the spleen of CORT treated mice. These studies are important since glucocorticoids are used clinically to suppress unwanted immune responses and to control inflammation and other symptoms associated with radiation and chemotherapy treatments.