In the rat uterus, glycogen is located in the myometrium (Bo & Atkinson, 1952) and is highest at pro-oestrus and after oestrogen stimulation (Boettinger, 1946). After a single injection of 1 or 10 μg oestrogen in ethanol, the glycogen concentration reached maximal levels at 14 h (Bitman, Cecil, Mench & Wrenn, 1965), while after injection of 10 μg oestrogen in oil, the polysaccharide concentration reached the highest levels at 24 h (Bo, Maraspin & Smith, 1967). Unpublished histochemical studies in our laboratory and the biochemical observations of Bitman et al. (1965) indicate that glycogen of the uterus does not increase more after repeated treatment with oestrogen than it does after a single injection. The results, therefore, indicate that with regard to glycogen, the uterus may become refractory to repeated treatment with oestrogen. In order to confirm the above findings and to form the basis for future work on glycogen synthetase: