The 42K method of Sapirstein has been modified to estimate the effect of gonadotropins on ovarian fractional blood flow. Microgram doses of luteinizing hormone (LH) cause a rapid increase in the proportion of the cardiac output delivered to the ovaries; the maximum increase obtainable, about 75 %, is present about 20 min after the injection of the hormone. An effect similar to that of LH is produced by histamine, but not by FSH, prolactin, serotonin, epinephrine, or norepinephrine. The fractional perfusion of the mature rat ovary is greatest during the estrous phase of the estrous cycle. Since LH administration also enhances ovarian 42K uptake in the mature rat, it is possible that one function of this hormone involves the cyclic regulation of ovarian blood flow. (Endocrinology75: 927, 1964)