Abstract: Traditionally, studies measuring behavioral activity have used male adult animals and grid crossings (GCs) as a representative measure of activity in lieu of total distance moved (TDM). However, using GCs as the dependent measure may not be effective for comparing the activity of animals during development, as they vary significantly in size. The present study examines the reliability of GCs as opposed to TDM as an indicator of locomotor activity for comparisons during ontogeny using a computerized behavioral tracking system (Noldus). Rats (postnatal day[PND] 35, PND 60) were tracked for a period of 3 minutes inside a closed runway. GCs and TDM were measured for the recorded tracks. It was determined that GCs were positively correlated with TDM in the behavioral apparatus, suggesting that GCs is a reliable measure of an individual animal's activity. Using GCs as the dependent measure, no significant differences in activity were observed across age or sex. However, using TDM indicates adolescent rats are significantly more active than their adult counterparts. These data indicate that although the number of GCs is predictive of total activity, the slope of the relationship varies significantly with age, therefore making it inappropriate to use GCs when comparing across ages. Studies that use animals of differing age must be sensitive to baseline differences in locomotor activity.