Rats were maintained on a thiamine-deficient diet to deplete skeletal muscle of thiamine pyrophosphate, and thus decrease the oxidative metabolism of pyruvate. The blood lactate concentration was significantly elevated in thiamine-deficient (TD) animals when compared with pair-fed (TP) controls. Analysis of diaphragm strips from these animals revealed that tissue lactate and pyruvate concentrations were not affected by any of the treatments employed. The rate of lactate efflux from TD tissues was, however, twice that from TP and 4.5 times that from weight-control (WC) tissues. The H+ efflux rate was also much greater in the TD muscle preparation than either of the control groups. Following 3 min of stimulation (150-Hz, 200-ms pulse train every 0.5 s), the degree of fatigue of tissues from each of these three treatment groups was not different. The observation in this study that glycolysis becomes the predominant metabolic pathway in thiamine deficiency without increasing the intracellular level of products, indicates that this treatment also has other effects which increase the effective lactate permeability of the fibre membranes.