A cDNA clone encoding equine follistatin was isolated from an equine ovarian cDNA library. Out of 1.2 x 10(5) independent clones screened, one positive clone was isolated and its cDNA sequence determined. The isolated clone, named EQ-FS-1, contained a complete open reading frame encoding 344 amino acid residues. The similarity of its deduced amino acid sequence to these of other mammalian species was greater than 95%. Although its expression level varied among the tissues examined, follistatin mRNA was detected in the equine uteroplacental tissues, follicles and corpora lutea by Northern blot analysis. In situ hybridization revealed that the expression of follistatin mRNA in the equine follicle was restricted exclusively to granulosa cells. When the expression pattern of follistatin mRNA in the equine uteroplacental tissues from mid- to late-pregnancy was examined, it was shown that its expression level tended to decrease after mid-pregnancy. These results suggest that follistatin acts in the reproductive tissues of the mare in maintaining pregnancy.