The ability to breathe against a mild expiratory obstruction was examined in 64 adult patients under halothane-nitrous oxide-oxygen anaesthesia. The trachea of all patients were incubated. It was found that d-tubocurarine 0.11 mg/kg blocked the respiratory musculature by 50 per cent. The onset of block was immediate, with the peak effect reached within one minute. The duration of block was either very short (1 to 2 minutes) or long (paralleling the blockade in the thumb twitch), depending upon the dose of d-tubocurarine. Following injection of a small dose (up to 0.1 mg/kg) the respiratory musculature might be blocked by 30 per cent but for only one minute, disproportionately short as compared with the thumb twitch. Following injection of a large dose (0.1-0.3 mg/kg) the duration was long, but the recovery was only slightly ahead of the thumb. Following an intermediate dose, a biaphasic pattern was observed. An initial partial recovery was observed during the first 1 to 2 minutes. Comparison of the respiratory musculature and the thumb twitch revealed that the respiratory musculature was highly sensitive to block and has a different time course. Succinylcholine 0.08 mg/kg blocked the respiratory force by 50 per cent with a time course similar to the block in the thumb twitch. The clinical significance of these observations are discussed.