Epidemiologic studies suggest an association between physical activity (PA) and breast cancer risk. We examined the relationship between voluntary wheel running and breast cancer in C3(1)/SV40Tag mice. Female FVB/N and C3(1)/SV40Tag mice were assigned to either PA [C3(1)-PA] (n=12) or sedentary (Sed) [C3(1)-Sed] (n=15) treatment and were placed in a cage with access to a running wheel (PA) or without (Sed) from 4 to 24 weeks of age (sacrifice). Physical activity data were analyzed for running distance, time and speed. Body composition was examined at 12 weeks of age. Tumors were counted twice weekly and at sacrifice to assess multiplicity. Tumor volume was calculated using external calipers [0.52 x (largest diameter) x (smallest diameter)2]. Heart and body weight were also recorded at sacrifice. Results showed that voluntary wheel running reduced tumor volume per tumor [C3(1)-Sed, 422.3±89.9 mm(3); C3(1)-PA, 260.2±61.7 mm(3)] (P<0.05), but was associated with increased tumor number (P<0.05). Body composition analysis showed no differences in body fat between the groups. Heart weight/body weight ratio was increased following physical activity (P<0.05) providing evidence of a training effect. In conclusion, voluntary wheel running activity was effective at slowing tumor growth in the C3(1)/SV40Tag mouse model of breast cancer, but did not inhibit tumor initiation. These data provide support for further development of the C3(1)/SV40Tag mouse model for use in understanding the role of physical activity on breast cancer progression and the mechanisms for its effects.