Hyperthyroidism is associated with muscle weakness, abnormal aerobic metabolism and increased lactate production. Muscle cell acidification during exercise is reduced, suggesting abnormally increased proton efflux. Using 31P magnetic resonance spectroscopy to measure cell pH and phosphocreatine concentration, we quantified effective proton efflux from rat leg muscle in vivo following 10-Hz sciatic nerve stimulation in seven rats injected with triiodothyronine (T3) for 5 days and in 11 controls. Proton efflux during recovery was sigmoidally pH-dependent in both groups but the initial proton efflux rate did not differ (16 +/- 3 mmol.kg-1.min-1 in treated animals vs 15 +/- mmol.kg-1.min.-1 in controls), despite significantly smaller pH change from basal in treated animals (0.60 +/- 0.04 vs 0.78 +/- 0.03 in controls, p = 0.002). The pH dependence of proton efflux can be characterized by an apparent Km, defined as the pH below basal at which proton efflux rate falls to half its start-of-recovery value. This Km was smaller in the T3-treated group (0.44 +/- 0.04 vs 0.59 +/- 0.03 in controls, p = 0.02). This suggests an increased affinity for protons by cell membrane proton transport processes such as the sodium-proton antiporter and may explain some of the metabolic changes seen clinically in hyperthyroid skeletal muscle.