Objective: The purpose of this study was to determine if the respiratory muscles of patients with COPD could be made to function anaerobically, as evidenced by an increase in arterial blood lactate concentration ([lactate](a)) during specific loading of the inspiratory muscles and, if so, the effect of a programme of high-intensity inspiratory muscle training on this function.Methods: In seven patients with COPD (FEV1 = 33 +/- 14% of predicted), measurements of [lactate]a were made each minute during progressive inspiratory threshold loading to voluntary exhaustion. These tests were performed before and after an 8-week programme of specific high-intensity inspiratory muscle training, combined with general whole-body exercise training.Results: During inspiratory muscle loading small increases in [lactate](a) (0.83 +/- 0.32 mM) were observed in two subjects before training, and in five subjects after training (0.69 +/- 0.57 mM). [Lactate], only increased when the inspiratory work rate exceeded 6.9 cm H2O L/min per kilogram of body weight, and when baseline maximum inspiratory pressure exceeded 65 cm H2O.Conclusions: The results of this study demonstrated that it is possible for COPD patients to increase inspiratory muscle work rate to a level requiring a major energy contribution from anaerobic glycolytic metabolism. This was only seen when inspiratory muscle strength and endurance were sufficient to allow it. Some patients who failed to demonstrate an increase in [lactate](a) at baseline did so after a programme of high-intensity inspiratory muscle training.