To observe a direct effect of 17beta-estradiol (E2) on thyroid activity, thyroid follicles were isolated from hypobranchial muscles of a freshwater murrel, Channa gachua. Thyroid follicles were incubated (5 X 10(6) follicles/well) in vitro at 30 degrees for 2 hr without hormone and then 3 hr with E2 or bovine thyroid-stimulating hormone (bTSH). Addition of 10 and 100 ng of E2 to thyroid follicles resulted in 2- and 3-fold increases in thyroxine (T4) release. When 100 ng of bTSH was added to check the isolated follicular function, it stimulated T4 release to more than 3-fold. Increasing doses of E2 from 1 to 100 ng caused a dose-dependent stimulation of T4 release, while a 1000-ng dose reduced T4 release compared to 100 ng. When thyroid follicles of this experiment were lysed by sonication and T4 content was determined, it corroborated the profile of T4 release in response to varied E2 doses. E2 was ineffective in increasing 125I uptake by the follicles, while bTSH elevated it by 45% over the control. Incubation of varied concentrations of [3H]estradiol with cytosol and nuclear fractions of thyroid follicles in the presence of a 100-fold excess of diethylstilbestrol showed saturable specific binding of E2.