Glycogen synthetase (EC 2.4.1.11) from oocytes and embryos of the loach (Misgurnus fossilis L.) has been found only in the D-form. The intensive glycogen accumulation during oogenesis did not correspond with the glycogen synthetase interconversion in the I-form.In isolated oocytes and embryos of the loach insulin transforms glycogen synthetase into the form which is desensitized for ATP inhibition. Insulin treatment enhancesV max without affecting theK m (UDP-glucose) only with an excess of activator-glucose-6-P. Simultaneously insulin treatment converts the phosphorylase (EC 2.4.1.1.) into the latent form.