Mammals exhibit social play, play fighting, and locomotor play. Locomotor play is defined as vigorous and seemingly purposeless behavior observed in young animals. Play can have negative consequences, such as taking time away from foraging, increasing exposure to predators, and causing injuries. One hypothesized benefit of locomotor play is facilitation of neural and muscular development in young individuals with effects that may persist into adulthood. Thus, animals from populations that have an evolutionary history involving natural or sexual selection for high levels of physical activity as adults might have evolved increased play behavior as juveniles as one mechanism that increases adult locomotor abilities. We tested whether elevated locomotor play has evolved as a correlated response in the context of artificial selection for increased voluntary exercise. We studied 4 replicate lines of mice that have been selectively bred for voluntary wheel-running behavior (High Runner or HR) and four non-selected Control (C) lines. Mice were sampled from generations 20 (2-7 days after weaning at 21 days of age), 67 (same), and 93 (15 days of age). Cages that included several individuals were the unit of observation (during the beginning of the scotophase, under red light). We recorded two forms of previously defined locomotor-play behavior: (1) very rapid, horizontally directed jerk-run sequences and (2) rapid ‘bouncing’ in a vertical direction. Preliminary statistical analyses indicate that HR mice may indeed have evolved increased levels of juvenile locomotor play. Supported in part by NSF grant DEB-1655362 to TG.