Testosterone (T) can be identified and quantitated by radioimmunoassay in peripheral serum and in the ovarian venous plasma of female rabbits. Peripheral serum T levels are low in ovariectomized rabbits and do not change following iv ACTH. In intact rabbits, levels of T and LH in the peripheral serum rise rapidly after mating, remain high for at least 3 hr, then drop to baseline values by 5 hr. Administration of either exogenous LH or synthetic LH-RH increases the concentration of both T and estradiol (E2) in the ovarian venous plasma, with T levels exceeding those of E2 about 10-fold. Although removal of surface Graafian follicles by thermocautery resulted in a drastic reduction in the ovarian venous concentrations of both steroids, the release of T (but not of E2) was markedly enhanced following an iv injection of LH-RH. These results suggest that whereas ovarian E2 production is largely confined to the follicle in the rabbit, T may be synthesized and released by both follicles and interstitial tissue. (Endocrinology94: 267, 1974)