We studied oxidative metabolism during bicycle exercise in 4 patients with myophosphorylase deficiency. Maximal oxygen uptake (VO2max) was low (14.0 +/- 1.4 ml X kg-1 X min-1, mean +/- SE) compared with that in normal subjects (37.7 +/- 1.9; n = 12) and patients with myalgia (24.9 +/- 1.8; n = 10). Carbohydrate oxidation, as estimated by the respiratory exchange ratio (R), was low relative to workload (max R, mean +/- SE: McArdle's disease, 0.96 +/- 0.02; normal subjects, 1.13 +/- 0.02; myalgia, 1.09 +/- 0.02). Intravenous glucose administration increased maximal oxygen uptake about 20% in those with McArdle's disease, but both VO2max and R remained lower than in control subjects. These findings suggest that the capacity for dynamic exercise in McArdle's disease is limited by the availability of oxidative substrate, and indicate that blood glucose is unable to substitute fully for muscle glycogen as an oxidative fuel. We also found that exercise cardiac output (Q) was excessive relative to oxygen uptake in affected patients (delta Q/delta VO2, mean +/- SE: McArdle's disease, 11.6 +/- 1.7; normal subjects, 4.8 +/- 0.2; myalgia, 5.6 +/- 0.2). This hyperkinetic circulation in exercise may serve to increase the delivery of blood-borne oxidative substrate to working muscle.