In the present work the effects of oestradiol benzoate (EB) on pituitary and plasma concentrations of TSH, plasma T4 and T3, and thyroidal activity of male and female rats have been studied. Wistar rats weighing between 150 to 200 g were injected sc with varying doses of EB in corn oil for 9 or 30 days. The animals were exsanguinated by cardiac puncture and the hypophyses removed and individually homogenized at 4 degrees C in 200 microliters PBS buffer. Pituitary and plasma TSH were measured by radioimmunoassay. Thyroidal activity was evaluated by a 4 h 131I uptake and by 48 h thyroidal release plasma slopes derived form the ratio PB[125I] (from thyroidal secretion) to PB[131I] (from exogenous [131I]T4). In both male and female rats the 10 and 25 micrograms doses of EB produced a significant decrease in pituitary TSH content; this effect was more pronounced when the 25 micrograms dose was given over 30 days. Plasma T4 decreased significantly; plasma T3 was moderately elevated in all groups (NS) and significantly increased in female rats treated with 25 micrograms EB (P less than 0.01). It is concluded that EB induced a marked depression of intrapituitary TSH, probably due to a decrease in synthesis, without affecting the release of TSH into the circulation. Moreover, EB accelerated peripheral T4 kinetics and thyroid gland activity, albeit to a moderate degree.