The loss of oestrogenic potency in aqueous solutions of dienoestrol and stilboestrol has already been reported by us [Wilder Smith & Williams, 1945], while the chemical instability of similar solutions was demonstrated independently by Warren & Gulden [1945]. The present paper describes the examination of some of the conditions affecting the stability of these and other oestrogens in solution. METHODS Preparation of solutions All solutions were prepared by dissolving 10 mg. of the oestrogen in 100 ml. of absolute alcohol or acetone. Aliquot portions of these solutions were diluted with the solvent to be investigated to give 1 μg./ml., and dilutions of these stock solutions were made at intervals for biological testing. A trace of toluene was added to the aqueous and urinary solutions to prevent bacterial or mould contamination. All except the urinary solutions were kept at room temperature. Biological assay The routine method used in this Institute was