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The juvenile stage of ontogeny is often characterized as a time of inactivity and quiescence for the immature reproductive system. The principle social behaviour by juveniles of many mammalian species is a rough-and-tumble activity known as play-fighting. An experiment is reported in which play-fighting by male rats was observed after various manipulations of gonadal steroids. Rats were housed in groups and castrated either on day 1 or day 10, times which are respectively during and after the sensitive period for androgeninduced organization of neural tissues in rats. Animals were injected with either 40 µg testosterone propionate or vehicle between 21 and 45 days of age, and play-fighting with unoperated, unfamiliar rats was examined. Castration on days 1 and 10 suppressed play-fighting. The behaviours remained suppressed in rats castrated on day 1 and injected with testosterone propionate, but testosterone propionate restored play-fighting to near-normal levels until 35 days of age in rats castrated on day 10. Moreover, gonadally intact juvenile males exposed to the androgen antagonist flutamide play-fought less than intact control males, although their social activity increased appreciably after day 35. It is concluded that the juvenile is not experiencing endocrine quiescence. Behavioural and physiological data suggest a reproductive system which is active, although differently from that in the adult. Hypersensitivity to testosterone and surges of gonadotrophin-releasing factors and LH at 35 days of age may be the events responsible for changes in play-fighting resulting from manipulations of gonadal steroids in juvenile rats.

J. Endocr. (1986) 110, 533–537