This study examined the day-to-day variability in voluntary wheel-running behavior among three genetically distinct lines of young male and female mice. Daily wheel revolutions were recorded at an age of 6-8 weeks in 10 males and 10 females from each of 3 lines: selectively bred line for high wheel running (Line 8), selectively bred for high wheel-running activity and fixed for a Mendelian recessive allele that reduces hind-limb muscle mass by 50% (Line 3), non-selected control (Line 2). There were significant mean differences in revolutions/day among weeks (P = 0.003), but the effect size was small (10%). Significant main effects for wheel running were also revealed for sex and line (P < 0.001). The grand mean +/- SD for the coefficient of variation (CV) of intra-individual wheel running was 23.0 +/- 10.8%. Although a significant main effect for the CV was found for week, the effect size was low (7%) (age 6 weeks, 23.4 +/- 10.9%; age 7 weeks, 25.1 +/- 13.2%; age 8 weeks, 20.1 +/- 7.8%). The overall mean CV was similar between females (21.4 +/- 9.8%) and males (24.4 +/- 12.0%) and among lines (Line 2, 23.4 +/- 9.8%; Line 3, 20.4 +/- 7.6%; and Line 8, 25.0 +/- 14.4%). These findings are consistent with our previous work in young humans and lend further support for the hypothesis that biological mechanisms influence daily levels of physical activity.