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The effect of estradiol - 17β (E2)treatment on the pituitary-thyroidal axis has been investigated in female Japanese eels in seawater. The eels were injected with 1 mg/kg body weight (BW)weekly for two to eight weeks; another group was treated with 5 mg/kg BW weekly for eight weeks and sacrificed three days after the last injection. In the pituitary, using RT- PCR system, the thyroid stimulating hormone β subunit mRNA (TSHβ mRNA) level is slowly increased in the low-dose E2 treatment group. A significant increase in values occurred after eight weeks E2 injections. The histological structure of the thyroid follicles suggests a hypo-activity as shown by a reduced epithelial cell height in eels injected with E2 after six and eight times, and eventually reached a minimum level after eight weeks. Using radioimmunoassay of thyroxine (T4), triiodothyronine (T3) in the serum, a slowly decrease of both hormones is demonstrated. On the other hand, the TSHβ mRNA level is significantly increased from initial in the high-dose E2 treatment group. The histological structure of the thyroid follicles suggests a hypoactivity, and a sharp decrease of the both hormones is also indicated. These data suggest that the expression of TSHβ mRNA level is increased and thyroid hormone release is depressed by E2, the effects depend upon E2 injection period and dose. Furthermore, E2 could not directly influence the TSHβ mRNA expression and, TSH transcription might be controlled by a negative feedback of thyroid hormone.