Sprague-Dawley rats (n=96) from postnatal 17-day-old to 24-day-old were compared for their exploration of 4 objects in an open-field, locomotor activity, object exploration time, habituation, and exploration time to the spatial change. As the locomotor activity increased, 23-day-old and 24-day-old rats explored the objects more than any other rats of the age groups. All rats showed habituation. It was not until the rats were 21-day-old that they re-explored the spatially changed object. 21-day-old rats more frequently investigated the object than older (22-day-old, 23-day-old, 24-day-old) rats. These results suggest that the exploration for objects precedes developmentally that of spatially changed object, and that 21-day-old rats begin to acquire informations about the spatial relationship in the environment
