A method for the preparation of high specific activity iodo-oestrogens is described together with the results of distribution studies of **1**3**1I iodo-oestradiol in the rat. It was concluded that the uptake of the radioactivity by the prostate was insufficient for localization studies by external detectors, and that in vivo, 2, iodo and 2,4 di-iodo-oestradiol released iodine, as suggested by the deposition of radioactivity into the thyroid. Data are presented in graphical and tabular form.