In a series of four experiments, social and experiential factors that influence the development of motor activity in rat pups were examined. Motor activity was monitored from postnatal days 13 to 21 as photocell interruptions in figure-eight mazes and comparisons were made between (1) pups maintained in a next box containing a dam and siblings and allowed access to the maze for 23 hr/day, (2) pups tested daily for 1 hr/day vs. pups tested only on postnatal days 15, 18, or 21, (3) pups tested daily for either 5 min, 30 min, or 1 hr/day, and (4) pups tested daily for 30 min/day either singly in a maze, paired with a littermate, or paired with an anesthetized pup of the same age. A monotonic increase in activity was seen for nest-box testing, minimal developmental change was seen for pups tested on only a single day or for pups tested with an anesthetized pup, whereas all other groups showed an inverted U-shaped profile of activity which was influenced by the duration of testing and/or the presence of a littermate. These data emphasize the relevance of environmental factors as determinants of preweaning behavior.