The effect of tubocurarine on the respiratory function of conscious, non-medicated volunteers was correlated with the degree of neuromuscular blockade measured with the train-of-four technique (the evoked response of the adductor pollicis brevis muscle to trains of four supramaximal ulnar nerve stimuli at 2 Hz, repeated once every 10 sec). Respiratory frequency, tidal volume, vital capacity, inspiratory force and peak expiratory flow rate were measured, (a) before administration of tubocurarine, (b) when the ratio of the amplitude of the fourth response to the amplitude of the first response of the train-of-four reached 60%, and (c) during recovery of the ratio until the control (100%) value was reached. Respiratory frequency, tidal volume and peak expiratory flow rate were not altered. Vital capacity and inspiratory force were both reduced significantly at the 60% level and the former also at the 70% level when compared with the control. The magnitude of change in all variables is of minor clinical importance, however, since the lowest measured values are well above acceptable minimum limits required for adequate respiratory function.