Cage stocking density can be manipulated by changing in cage size and group size in the cage. The effects of these two factors were investigated in three developmental stages in male rats: juvenile, post-pubertal, and adult. An open-field test was conducted to measure exploratory behavior which was influenced by levels of activity, anxiety, and exploration. When the cage size was decreased, juvenile rats displayed decreased locomotion and lower propensity for exploration, whereas such changes were not evident in post-pubertal rats. When the group size was increased, adult rats exhibited higher locomotion. However, these factors did not affect risk assessment behavior of rats in these developmental stages. Thus, it appears that the effect of stocking density differs depending on the developmental stage of the animal: Juvenile rats increased anxiety following limiting space, whereas adult rats increased activity following increase in social tension.