Much work over the last several years indicates the neonatal rat testes elaborates a substance presumably responsible for hypothalamic sexual differentiation. Furthermore, additional studies indicate this differentiating action may be mimicked by testosterone.

The present study was undertaken to establish whether, in fact, labeled testosterone or estrogen administered subcutaneously could be detected in the neonatal rat hypothalamus.

Rats 1, 2, 3, 5, 10 and 20 days old were given microcurie amounts of a labeled steroid. One hour later milligram amounts of hypothalamus, cerebrum, cerebellum, gonad, liver and muscle tissuewere taken for determination of radioactivity. Assey for days 1, 2, 3, and 5 showed 1, 2 –H3 testosterone localized primarily in the hypothalamus; much less in the cerebrum and liver. On days 10 and 20 hypothalamic uptake was significantly less; liver uptake significantly greater. Cerebellar, gonad and muscle tissue contained minimal activity all days. Autoradiographid studies corroborated these findings. Comparable estrogen studies seemed to show less time dependency.

These data indicate that during certain days (critical period) the hypothalamus is selectively receptive to labeled androgen.

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