The social play of pairs of juvenile rats can be brought under tight experimental control using social deprivation, and it can be objectively quantified by measurement of pinning behavior. Research and conceptual issues concerning this paired-encounter procedure are summarized, including issues related to (1) measurement, (2) gender differences (and the absence thereof), (3) relations between play and aggression, (4) the regulatory processes interacting with and underlying play, (5) the neurochemical and neuroanatomical substrates of play, (6) the functions of play in dominance and other adult behaviors. Existing results suggest the operation of a harmoniously operating brain process which generates a unique emotive brain process that is appropriately referred to as social play. Although the concept of play remains to be adequately defined, the position is advocated that rigorous psychobiological analysis will ultimately provide an empirical definition based upon neural circuit characteristics. Analysis of the underlying circuits may help reveal the manner in which more complex levels of behavioral competence arise ontogenetically, and work in the area may yield clues to the genesis of several psychopathologies.