Five-day-old female rats were androgenized with a single dose of 100μg (TP100A) or 1000μg (TP1000A) testosterone propionate. The androgenized adult rats together with normal rats were tested for prolactin and TSH responses to synthetic TRH. Five μg/kg of TRH was injected iv4times at2hr intervals, and serum samples were obtained5min after each injection. The prolactin release by exogenous TRH in normal rats was less marked. In TP1000A rats, the basal prolactin level in the serum was higher than that in normal or TP100A rats and a further elevation was induced by the1st injection of TRH. In TP1, 00A rats, the serum prolactin level increased five-fold more than the maximal level in normal rats by the1st and 2nd injections, while the pituitary prolactin content showed a less marked decline and a tendency to increase after the4th injection, suggesting an enhanced release and production of prolactin by TRH in TP100A rats. TRH injection stimulated TSH release in androgenized rats as well as in normal rats. The magnitude in the TSH release was TP1000A>normal≥TP100A. The TSH contents in the pituitary after TRH injection tended to decrease in TP1000A and normal rats but not in TP100A rats. In TP100A rats, increases of TSH content were evident by the3rd and4th TRH injections. The weight of the anterior pituitary decreased in normal and TP1000A rats but tended to increase in TP100A rats after the treatment with TRH. The present results indicate that not only lactotrophs but also thyrotrophs in the pituitary became very sensitive to exogenous TRH by neonatal androgenization. A marked difference in the responsiveness to exogenous TRH between TP100A and TP1000A rats could be due to the fact that the alterations in the hypothalamus and secondarily in the ovary were induced by testosterone propionate in the dose-dependent manner.