Anterior pituitary mRNA was prepared using a microscaled method and translated in a wheat-germ cell-free system in the presence of [35S] labeled cysteine and methionine. Translation products, immunologically related to LH beta, FSH beta and the common subunit alpha, were isolated as precursors with antisera to denatured subunits and characterized by SDS-polyacrylamide gel electrophoresis and fluorography. The radioactive bands were excised from the gel and counted for quantitative evaluation. Our data show that translation of pituitary mRNAs from ovariectomized (ovx) rats results in precursor levels increased by 10 fold for alpha and 14 fold for LH beta as compared to the levels in normal rats. The increase in FSH beta precursor was impossible to evaluate as a specific immunoprecipitation product was undetectable in the case of normal rats. Estradiol, but not progesterone, administered in vivo to ovariectomized rats, reversed the stimulatory effect of ovariectomy on the expression of mRNAs coding for gonadotropin subunit precursors 48 h after injection. These results suggest that estradiol, but not progesterone, negatively regulates the synthesis of the pituitary gonadotropins in the female rats via changes in specific mRNA levels. This hormonal control probably occurs at the transcriptional level.