It has been shown by Dodds, Golberg, Lawson, and Robinson [1938] that certain di-esters of diethylstilboestrol have a prolonged action when injected into spayed rats, as have the esters and di-esters of naturally occurring oestrogens. Maximum prolongation of oestrus occurred, however, with the dipropionate, not with the higher esters (dibutyrate and divalerate) of the fatty acid series. This is in contrast to the behaviour of the esters of oestrone and oestradiol, of which the higher esters have a more prolonged action than the lower ones [Miescher, Scholz, and Tschopp, 1938 a, b, c]. The dibenzoate and dimethyl ether of diethylstilboestrol also had prolonged oestrogenic activity when given in doses of 100 μg. or more. Robson, Schönberg, and Fahim [1938], using criteria other than full cornification, have reported that large doses of oestradiol benzoatebutyrate (500 μg.), of triphenyl ethylene (10,000 μg.), and of triphenyl chlorethylene (500 μg.), injected into spayed mice