Abstract Glycogen synthetase I and D were purified 250 and 150 fold respectively from pig brain. In crude fractions both enzymes were associated with glycogen and were fully active in the absence of added glycogen primer. Glycogen was removed during purification, and the purified enzymes were only partially active when assayed in the presence of primer. Preincubation of the purified enzymes with glycogen resulted in a 6-fold activation of synthetase I and a 3-fold activation of synthetase D; no primer was required in the assay mixture. Glucose-6-P increased the rate and extent of activation of both forms of synthetase at 23° and at 37°.