This project was designed to determine the genetic (between-strain) and environmental (within-strain) variance in daily running wheel activity level in inbred mice. Five male and five female mice, 9.7-15.3 wk old, from each of 13 strains (A/J, AKR/J, BALB/cJ, C3H/HeJ, C57Bl/6J, C57L/J, C3Heb/FeJ, CBA/J, DBA/2J, SWR/J, MRL/MpJ, SPRET/Ei, and CAST/Ei) as well as five female NZB/BinJ mice were housed individually. A running wheel in each cage was interfaced with a magnetic sensor to measure total daily distance and exercise time for each animal every 24 h for 21 consecutive days (3 wk). Average daily distance (km), duration (min), and velocity (m/min) for each strain was then calculated. Significant interstrain differences in average daily distance (P < 0.001), average daily exercise duration (P < 0.0001), and average daily exercise velocity (P < 0.0001) were found, with C57L/J mice running farther and faster than the other strains. Sex was a significant factor in daily running wheel activity, with female mice running an average of 20% farther (P = 0.01) and 38% faster (P < 0.0001) than male mice. The male mice ran 15% longer duration on a daily basis (P = 0.0091). Weight was only associated with exercise velocity in the female mice, but this relationship was not significant when subdivided by strain. Broad-sense heritability estimates on the physical activity differed by sex (for distance, male 31-48% and female 12-22%; for duration, male 44-61% and female 12-21%; for velocity, male 49-66% and female 44-61%). In conclusion, these data indicate that daily running wheel activity level in mice is significantly affected by genetic background and sex.