Binding of tritiated iodinated gonadotropins (LH, HCG, FSH, or prolactin) to ovarian cells can be measured in ovarian tissue after in vivo injection of the hormones or after in vitro incubations of slices or isolated cells or homogenates. Studies are meaningful only if the labeled hormone is in a biologically active state. FSH binds almost exclusively to granulosa cells of medium size and large follicles. LH and HCG appear to share the same receptor and bind primarily to corpus luteum tissue, thecal cells of large follicles, and to granulosa cells. Thecal tissue isolated from large preovulatory porcine follicles binds more HCG compared to thecal tissue isolated from adjacent small follicles. This difference in receptor number (or affinity) may explain why only the thecal tissue of large follicles can respond to the preovulatory surge of LH by ovulating. Granulosa cells from large porcine follicles bind 10- to 1000-fold more HCG, compared to the granulosa cells harvested from small or medium-sized follicles, indicating that the maturational state of the follicle determines the number of LH-HCG receptors on the granulosa cell. This accounts for the observation that granulosa cells from large follicles luteinize in culture and after ovulation in vivo, whereas granulosa cells obtained from small follicles do not luteinize even in the presence of exogenous LH. The factor(s) controlling induction of the gonadotropin receptors in the thecal as well as granulosa cells as the follicle matures are unknown. Stromal tissue binds little or no gonadotropins. It therefore can be generally accepted that the characteristics of receptors determine which cellular component of the ovary will respond to gonadotropins.