The localization of transforming growth factor-beta 1 in the fetal and neonatal rat testis (from day 13.5 of fetal life to postnatal day 20) was investigated by an immunohistochemical staining method employing a polyclonal anti-TGF-beta 1 antibody that does not cross react with either TGF-beta 2 or TGF-beta 3. In testis and mesonephros tissue, immunostaining for TGF-beta 1 was undetectable on fetal day 13.5 and appeared exclusively in the primordial Sertoli cells on fetal day 14.5. Staining in Sertoli cells was still clearly observed on days 15.5 and 16.5 of fetal life and became faint from fetal day 18.5 onwards. In fetal Leydig cells, a positive reaction for TGF-beta 1 appeared on day 16.5 and became very intense during late fetal life. After birth, fetal-type Leydig cells, which were still observed on postnatal days 4 and 20, also exhibited a very strong immunostaining for TGF-beta 1, whereas adult-type Leydig cells, observed on day 20, showed a slight staining. No immunoreactivity for TGF-beta 1 was found in germ cells and peritubular cells on any day studied. In conclusion, TGF-beta 1 is present very early in the fetal rat testis and its prevailing localization shows age-related changes, which suggests that this factor plays an autocrine/paracrine role in the regulation of testicular function and differentiation, during early development.