This thesis investigated the effects of stress and different exercise conditions on the promotion/progression of MAT-B mammary tumor cells in vivo. In experiment one, it was demonstrated that stress, associated with the use of urine collection devices (UCD), caused significantly greater incidence (p<0.05), increased volume (p<0.01), and greater weight (p<0.005) and growth rate (p=0.0001) of primary tumors in urine collection (UC) animals (n=12) compared to non-UC animals (n=19). It is hypothesized that the elevated glucocorticoid levels in the UC group caused a Th1/Th2 shift in the immune system and that this resulted in the differences in tumor parameters observed. In experiment two, the effect of different exercise conditions were investigated. Forced exercise (FE) (n=10), a moderate intensity exercise, was achieved by treadmill-running and voluntary exercise (VE) animals (n=10) had running wheels in their cages. Sedentary (S) animals (n=10) served as the control group. Results indicate that FE animals had significantly slower primary tumor growth rates and their median time to sacrifice was significantly longer than the S animals (p<0.05). It is believed that FE modulates the Th1/Th2 shift induced by the UCDs.