Three variables constituting an index of rearing behavior from an animal-activity monitoring system were compared with a standard visual rating at various ages of neonatal and adult rats. All three automated measures correlated significantly with measures rated by visual observation, although only the "number of vertical episodes" was similar in magnitude to the actual number of observed rearings. The height of the vertical sensors was critical in detecting rearings, although reliable results were obtained over a relatively wide range of heights. Analyses in neonatal rats demonstrated developmental progression in rearing behavior, with an onset around Day 15 of age. This automated technique was found to be useful and eliminated confounding variables, such as experimenter bias, commonly associated with visual observation techniques.