Adolescence and young adulthood may be critical windows in establishing risk for breast cancer development in humans. Epidemiological data suggest that exercise during this life stage is associated with decreased breast cancer risk yet few experimental studies to elucidate the mechanism have been performed. The purpose of these studies was to evaluate the effects of moderate exercise training on mammary tumour development in adolescent rats using the 1-methyl 1-nitrosourea (MNU) chemical carcinogen model. Exercise (EX) consisted of moderate-intensity treadmill running 30 min/day, 5 days a week. A total of 274 animals were used: 94 in study 1 and 180 in study 2. Animals were injected with MNU (50 and 25 mg/kg body weight in studies 1 and 2, respectively) at 21 days of age and began training at 28 days of age. Groups of animals (n=10-30 depending on the study and time point) were sacrificed every 2 weeks for 8 weeks to evaluate tumour development. No difference in median tumour-free survival time was observed in the EX versus sham-exercise (SHAM), nor were there any differences in multiplicity at either a high or moderate dose of MNU. Latency to first tumour palpated was increased in both studies by 3-4 days. Consistent across both studies, tumour weights were less and the growth rates of the tumours, defined as tumour weight divided by the number of days elapsed since the tumour was first palpated, were reduced in the EX group. The data suggest that latency is increased and tumour growth is retarded in response to moderate exercise training.