The metabolism of gonadotropins was unclear until the 1960s. The chief theory, utilization of gonadotropins by gonads, was unproven, but radioimmunoassay indicated that the levels of luteinizing hormone entering the ovary were higher than the levels in the ovarian veins. The availability of radiolabeled proteins opened the possibility of following the fate of gonadotropins in the end organ. Independently, two teams in Tel Aviv and Seattle researched the uptake of radiolabeled human chorionic gonadotropin by rodent ovary. Both concluded that the ovary bound gonadotropin; however, neither pursued the mechanism of the observation, gonadotropin receptors on ovarian cells. Had they done so, the course of discovery and study of cell surface receptors might have been altered.