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Summary. Estradiol (E 2 ) level in plasma of non-breeding female Tilapia aurea, measured by RIA, was 0.33: : :!:: 0.02 ng/mi (mean ± SEM, n = 6). This level is about 10 p. 100 of the level found in breeding females, about 4 times higher than in ovariectomized females and similar to the level found in non-breeding males.Twenty-four hours following a single i. p. injection of 0.5 mg E 2 , the estradiol level in plasma of ovariectomized Tilapia was within the physiological range (0,2-10 ng/mi). This level was maintained for at least 4 days. Comparison of the vitellogenic potency of 3 estrogens in ovariectomized T. aurea shows that estriol was more potent than estradiol, which, in turn, was more potent than estrone. Since E 2 increased in the plasma of estrone-injected fish, it is assumed that estrone was converted to estradiol in the recipient fish.Introduction.