To test the hypothesis that testosterone (T) sexually differentiates gonadotropin secretion and sexual behavior by inducing synthesis of messenger RNA (mRNA), newborn female rats received intrahypothalamic infusions of saline or cordycepin, an adenosine analogue that preferentially inhibits synthesis of polyadenylated mRNA, an hour before they received T propionate (TP) systemically. As adults, controls were anovulatory and did not become sexually receptive when given estradiol benzoate (EB) and progesterone (P). Cordycepin-treated females obtained lordosis quotients (LQs) three times those of controls and most of them ovulated. Cordycepin also curtailed the defeminizing effects of some doses of moxestrol, an artificial estrogen; thus it does not simply block aromatization. Some groups were retested for lordosis using EB and methysergide to bypass P receptors. Methysergide increased LQs in one group that received moxestrol + cordycepin as neonates and that was moderately responsive to P, but it did not increase sexual receptivity among the saline-treated controls. These data suggest that defeminization of sexual behaviour involves more than defeminization of P receptor synthesis.