Juvenile social play behavior is one of the most important developmental behaviors for many animals, including humans. Social play facilitates the development of a variety of adult behaviors, including adult sex behavior and aggression. In this study, we redistributed the rat pups into home cages that were either single-sex female, single-sex male, or mixed sex on postnatal day (PN) 13. For this preliminary report, juvenile play behavior was observed in both focal observation and paired encounter paradigms from PN25-PN30. We hypothesized that the sex composition of the litter would have an effect on juvenile play behavior and our preliminary analyses support this hypothesis.