Previous works from this laboratory have demonstrated that oestradiol benzoate (EB) in euthyroid male and female rats induced a significant decrease in the pituitary content of TSH while serum levels of this hormone remained normal. The present work studied the effects of EB (25 micrograms/100 g body weight, during 9 days) on the peripheral metabolism of [125I]rTSH and on the pituitary and plasma concentration of TSH in euthyroid and hypothyroid rats. No significant variations were observed in [125I]rTSH kinetics of EB-treated euthyroid rats vs untreated controls: fractional turnover rate 2.8 +/- 0.2 vs 3.0 +/- 0.3%/min, distribution space 6.5 +/- 0.4 vs 6.8 +/- 0.5 ml/100 g body weight, disposal rate 18.4 +/- 2.4 vs 18.1 +/- 1.9 microU/100 g/min and extrapituitary pool 645 +/- 42 vs 614 +/- 43 microU/100 g body weight. Similarly, in hypothyroid rats oestrogens induced no changes in TSH kinetics except for an increase in distribution space (P less than 0.025). However, oestrogens decreased the pituitary pool of TSH (P less than 0.001) in both euthyroid and hypothyroid rats and increased the plasma TSH in hypothyroid animals (P less than 0.01), all vs their respective controls. Neither hypothyroid group had detectable plasma levels of T4 and T3. In summary: 1) the marked decrease of pituitary TSH with normal plasma TSH induced by EB appears unrelated to the peripheral metabolism of TSH, 2) the results from hypothyroid rats suggest that EB stimulates the release of TSH from the pituitary gland.