1. Kinetic properties of adult Phormia fat body glycogen synthetase were studied and compared to other animals. The KM for UDPG is 2.82 mM, decreasing to 0.58 mM in the presence of G-6-P. 2. The specific activity of fat body glycogen synthetase shows a reduction of 30% within 2 days after allatectomy. 3. Fat body T-6-P synthetase activity decreases to 70% of the control value after cardiacectomy. 4. Corpus cardiacum homogenate fails to induce higher T-6-P synthetase activity in cell-free preparations from cardiacectomized flies. 5. Interactions between corpus cardiacum and corpus allatum in regulating carbohydrate metabolism are discussed.