Progesterone (P) and 17 beta-estradiol (E) inhibit transcription of alpha mRNA (alpha mRNA) by about 70% in ovine pituitary cultures. In addition, P, but not E, shortens alpha mRNA during the 3-24-h period following treatment. Data presented here focus on this post-transcriptional effect of P, which appears totally independent from transcriptional changes and may alter the translational efficiency of the affected mRNA or its half-life. Results indicate this shortening occurs at the poly(A) tail. The normal length of this tail was estimated to be 70-100 nucleotides in the case of alpha mRNA, and it was shortened by 30-40 nucleotides during P treatment. Shortening was normally observed after 3-10 h of P treatment and was complete by 10-24 h. Significant shortening occurred at 7-10 nM P in ovine pituitary culture; the 10-15-nM concentration of P in the serum of luteal phase ewes also appears sufficient to shorten alpha mRNA in vivo since alpha mRNA from the pituitaries of these ewes was found to be consistently shorter than alpha mRNA from ovariectomized ewes. Finally, P also shortened the poly(A) tail of mRNA for the beta-subunit of LH. This indicates that P-induced poly(A) shortening not only affects alpha mRNA but may affect several or many mRNAs found in ovine gonadotrophs. The significance of poly(A) shortening is not yet understood, but it could involve post-transcriptional inhibition, which, combined with transcriptional effects, may make P a more potent inhibitor of gonadotropin synthesis than E.