Objective - To determine concentrations of proglycogen (PG), macroglycogen (MG), glucose, and glucose-6-phosphate (G-6-P) in skeletal muscle of horses with polysaccharide storage myopathy (PSSM) before and after performing light submaximal exercise. Animals - 6 horses with PSSM and 4 control horses. Procedures - Horses with PSSM completed repeated intervals of 2 minutes of walking followed by 2 minutes of trotting on a treadmill until muscle cramping developed. Four untrained control horses performed a similar exercise test for up to 20 minutes. Serum creatine kinase (CK) activity was measured before and 4 hours after exercise. Concentrations of total glycogen (Gt), PG, MG, G-6-P, free glucose, and lactate were measured in biopsy specimens of gluteal muscle obtained before and after exercise. Results - Mean serum CK activity was 26 times higher in PSSM horses than in control horses after exercise. Before exercise, muscle glycogen concentrations were 1.5, 2.2, and 1.7 times higher for PG, MG, and Gt, respectively, in PSSM horses, compared with concentrations in control horses. No significant changes in Gt, PG, MG, G-6-P and lactate concentrations were detected after exercise. However, free glucose concentrations in skeletal muscle increased significantly in PSSM horses after exercise. Conclusions and Clinical Relevance - Analysis of the results suggests that glucose uptake in skeletal muscle is augmented in horses with PSSM after light exercise. There is excessive storage of PG and MG in horses with PSSM, and high concentrations of the 2 glycogen fractions may affect functional interactions between glycogenolytic and glycogen synthetic enzymes and glycosomes.