Male and female rats were injected with oestradiol benzoate, 1 mg twice a week. In the diet they received 0.034%<sub>o</sub> clomiphene citrate, SQ 10.591 and MER 25, the administration of the oestrogen being combined with the above-mentioned anti-oestrogens In male rats, 3 weeks administration of clomiphene did not cause a change, SQ 10.591 caused a slight increase and MER 25 a slight decrease in the weight of adenohypophysis. Clomiphene slightly reduced, SQ 10.591 did not affect, and MER 25 reduced considerably thyroxine binding to adenohypophysial proteins in vitro. Hypertrophy of the adenohypophysis after oestrogens was somewhat prevented by clomiphene and less by SQ 10.591 and MER 25. The increase of thyroxine binding to adenohypophysial proteins in vitro after oestrogen was influenced in a similar way. Clomiphene and SQ 10.591 increased the weight of the adrenals and reduced the weight of the testes. The weight of the seminal vesicles declined most after clomiphene, less after SQ 10.591 and least after MER 25. In female rats the weight of the adenohypophysis increased considerably after oestrogen, declined slightly after clomiphene and did not change after the other two anti-oestrogens. None of the administered anti-oestrogens prevented the increase in the weight of the adenohypophysis after oestrogen, and with the exception of MER 25, they did not prevent the increase of thyroxine binding to adenohypophysial proteins in vitro. Under the above experimental conditions (large dose of oestrogen), clomiphene seems to be the most effective anti-oestrogen and MER 25 the purest one, i.e. the anti-oestrogen most devoid of oestrogenic activity.