Lesions of either the dorsomedial thalamus (DMT) or the parafascicular region of the thalamus (PFA) reduced rough-and-tumble social play in juvenile rats, as measured by frequency of pinning. Pinning was reduced by 33% in pups with DMT lesions and by 73% in pups with PFA lesions. Although PFA lesions had minimal effects on average pin durations, DMT pups had pins that were, on average, 105% longer than those of controls. Lesions of the PFA, but not DMT, also reduced play solicitation behaviors. Pups with PFA lesions were also insensitive to the play modulating effects of both naloxone and morphine. DMT pups were sensitive to the facilitatory effects of morphine, but naloxone was without effect in these animals. Control studies designed to evaluate general behavioral competence showed that the observed lesion effects were relatively specific to play.