Three enzymes of adenine nucleotide metabolism, adenylosuccinate synthetase (ASS), adenylosuccinate lyase (adenylosuccinase, ASL), and AMP deaminase (AMPDA), form the purine nucleotide cycle.1,2 Deficiences of muscle AMPDA (often called myoadenylate deaminase in the clinical literature) are frequently diagnosed either as a primary genetic, or as a secundary defect in patients presenting with muscular symptoms.3,5 In contrast, very few patient studies have been devoted to the two other enzymes of the purine nucleotide cycle. Only Kar and Pearson6 reported normal activities of ASL in muscle from patients with Duchenne and other neuromuscular diseases. In the present study ASS, ASL, and AMPDA were assayed in a series of muscle biopsies taken from patients suffering from fatigue and cramps following moderate to vigorous exercise.