Follistatin (FS) or FSH-suppressing protein is a polypeptide which exists in multiple forms and has inhibin-like activity. We investigated the distribution of tissues expressing FS mRNA. A segment of the rat FS mRNA corresponding to the last 157 nucleotides of exon 5 and the first 71 nucleotides of exon 6 was prepared by PCR and subcloned in plasmid GEM4Z (pGEM4Z) to produce a radiolabeled RNA probe. S1-Nuclease analysis of RNA extracted from rat tissues indicated alternative splicing of the rat FS gene and gonadal as well as extragonadal expression. In addition to the ovary and testis, the FS mRNA was detected in cerebral cortex, pituitary, adrenal, thymus, pancreas, gut, kidney, heart, uterus, skeletal muscle and lung. Treatment of female rats with combined Pregnant Mare Serum Gonadotropin (PMSG) and human Chorionic Gonadotropin (hCG) increased expression of FS mRNA in the ovary. Our results show that the mRNA for the precursor of the larger known form of FS (FS 344) is much more abundant than the mRNA for the smaller molecule (less than 5%) and that stimulation of FS transcription by PMSG and hCG in the ovary does not change this relationship.