SUMMARY: Ovarian tissue of prenatal, newborn, and 5-day-old rats does not specifically bind 125Ilabelled HCG. Specific binding of HCG was first observed in ovaries of 10-day-old animals and binding increased with age. These results indicate that, contrary to rat testis, the HCG receptor in the rat ovary is not present during foetal and early postnatal development. Thus, the insensitivity of the ovary to endogenous and exogenous LH or HCG during this developmental period is due to the lack of specific receptors.