Follistatin (FS), a monomeric glycoprotein which specifically binds activin, is expressed in many tissues. This study investigated 1) the ontogeny of the steady-state FS mRNA levels in different extragonadal tissues and 2) whether the ratio of the differential splicing products, FS 344 or its carboxy-truncated form FS 317, is changed during postnatal development. Whereas the levels of FS mRNA 344 in the kidney showed a profound increase from the day of birth to adulthood, the levels in the muscle peaked during the infantile period and then declined. Brain cortex, heart and thymus also showed tissue specific expression in the steady-state mRNA level of FS during postnatal development. None of the tissues showed a measurable change in the ratio of the mRNA for FS 344 and FS 317. The FS mRNA 344 levels in male and female kidney were not different. It is concluded that the ontogeny of steady state FS mRNA varies in a tissue specific manner during postnatal development of the rat and may be involved in modulating the outcome of activin.