Glycogen phosphorylase (a-1,4-glucan: orthophosphate glucosyltransferase) and glycogen synthetase {UBPG:a-1,Q-glucan a-4-glucosyltransfepase) have been examined in red and white skeletal mmcle of the guinea pig. Histochemically phosphorylase was foulid to be more active in white than in red muscle fibers but no difference in glycogen synthetase could be detected between the fiber types. H s~~v e r , quantitative determinations showed that total glycogen synthetase activity (I + D) was higher in red than in white muscle (1.48 f 0.14 S.E.M. vs. 0.71 f 0.09 yrnslee/minute per g wet weight a t 37'). The converse reIatiotnship held for total phosphorylase activity (a -+ +b), which was greater in white than in red muscle (19.24 f 2.93 vs. 10.43 f 2.84 Irmoles/minute per g wet weight a t 38"). The phosphsr$ase a level of 3.63 f 8.96 in red muscle a t rest was simiiar to that of 5.44 f 1.19 in resting white muscle. Stimulation produced a significant csnversion of phosphsrylase b to a only in white muscle. After 30 seconds stimulation with 1-volt impulses of 20 milliseconds duration at a rate of 20 pulses per second, the phosphory-lase a activities of red and white muscle were respectively 3.72 f 1.88 and 16.66 f 1.79. After stimulation the glycogen synthetase values in white and red muscle were 1.82 % 0.07 and 1.72 f 0.11 respectively. lSupprted by grants from the C a d i a n Muscular Dystrophy Association and the Medical Research Council of Canada. This work was submitted by S. St. George Stubbs in partial fulfillment of the requirements f a the degree of B.Sc (Med.).