This work was originally undertaken in order to elucidate certain features of the structure of the substance known as "sodium oestriol monoglucuronidate" isolated by Cohen, Marrian and Odell (1936), and to investigate its purity. The work was beset by difficulties and delays, particularly the low yields in the preparation of purified NaOG. These may be attributed in part to the difficulty in obtaining fresh supplies of genuine late pregnancy urine from the busy wards of the Maternity Hospital. Consequently attention was diverted to other problems¡ which could be investigated while supplies of NaOG accumulated.