Inhibin is a gonadal hormone composed of an a-subunit and one of two beta-subunits (betaA, betaB), and its primary role is to inhibit FSH secretion by the pituitary. To investigate the roles of inhibin alpha in the reproductive system, an expression plasmid, pCMV-rINA, with the rat inhibin alpha cDNA fused to the cytomegalovirus promoter, was introduced into muscle by direct injection. Inhibin alpha mRNA was detected in the muscle by RT-PCR and Southern blot analysis. Inhibin protein was also detected, and Western blot analysis revealed a relatively high level of serum inhibin, but not of activin betaA. The estrous cycle of the pCMV-rINA-injected mice was extended, but there was no change in levels of pituitary FSH mRNA or serum FSH and no ovarian cysts were observed. When injected female mice were mated with males of proven fertility, litter size increased. Surprisingly, the embryos of pregnant females injected with pCMV-rINA, were retarded in growth and had defects in internal organs. When male mice were injected, testicle weight increased slightly without any noticeable change in the histology of the seminiferous tubules. Taken together, our data indicate that the inhibin alpha subunit influences a number of the reproductive functions of female mice.