The effect of oestrogens on the uterus of rats and mice has been evaluated in different ways, as by observing changes in uterine weight (Astwood, 1938; Lauson, Heller, Golden & Sevringhaus, 1939), number of mitoses (Tice, 1961), nuclear volumes (Gelfant & Clemmons, 1955), or the thickness of the different tissue layers (Palkovits, Palkovits & Czeizel, 1963). Although these methods give numerical parameters for the effect of oestrogens, they are rather elaborate, and for a rapid screening of the influence of different oestrogens or anti-oestrogens on the uterus, a more simple method would be useful. The present paper describes a morphological test of the reaction of the mouse uterus to oestrogen which is easy to perform but permits only a subjective measurement of the effect. The acriflavine method An injection of oestrogen into a spayed mouse initially causes water imbibition by the uterus which can be visualized histologically, if the accumulating