OBJECTIVE To determine whether polysaccharide storage myopathy (PSSM) in Quarter Horses is attributable to a defect in glycolysis or in the allosteric regulation of phosphofructokinase (PFK) enzyme. METHODS Muscle biopsy specimens were obtained from 6 Quarter Horses with PSSM and 8 Quarter Horse or Thoroughbred control horses. METHODS Maximal activity of glycogenolytic and glycolytic enzymes was determined spectrophotometrically. Maximal activity of PFK was determined for each horse at pH 8.0, and at pH 7.0 when variable concentrations of the activators, fructose 6 phosphate, fructose 2,6 bisphosphate, and adenosine monophosphate or inhibitors adenosine triphosphate and citrate were added to the reaction mixture. Relative activity was calculated as activity at pH 7/maximal PFK activity. RESULTS Deficiencies in glycogenolytic or glycolytic enzyme activities were not evident in horses with PSSM. Differences between horses with PSSM and control horses in relative activity of PFK were not apparent for any of the activators or inhibitors used in the study. CONCLUSIONS In a group of horses with PSSM, we were unable to detect a glycogenolytic or glycolytic enzyme deficiency or abnormality in the allosteric regulation of PFK. CONCLUSIONS Although PSSM is clinically and histologically similar to glycogenolytic/glycolytic enzyme deficiencies in human beings and other animal species, abnormalities in this metabolic pathway are not present in horses with PSSM.