The effects of altering neonatal levels of progestins on the later development of social play behaviour was studied. Progestin levels were raised in experiment one by administering injections of either progesterone or medroxyprogesterone acetate. This indicated that exposure to either hormone led to reduced levels of social play in juvenile rats of both sexes, confirming earlier reports of lowered levels of play following medroxyprogesterone obtained via maternal milk. In Experiment 2, endogenous progestin levels were lowered by administration of the antiserum to progesterone. The prediction that this should result in raised levels of juvenile play was supported for males, but not for females. Females in Experiment 2 by contrast showed a decrease in play. Possible reasons for this sex difference in response to progesterone antiserum are discussed.