SUMMARY.The interconversion of oestrone and oestradioI-17^ occurs in human and rat blood cells and to a lesser extent in plasma. Reduction of oestrone produces only oestradiol-17;3 which has been identified by preparation of its pipsylate. This can be separated from the pipsylate of oestradiol-17a by paper chromatography. Oestrone and oestradiol-17^ also yield polar substances when incubated with blood preparations.The preparation and constants of a number of oestrogen pipsylates are described.INTRODUCTION.