To quantify the neuromuscular blocking effect of vecuronium on the diaphragm and the adductor pollicis, single twitch stimuli were applied to the phrenic nerves at the neck and the ulnar nerve at the wrist in anaesthetized patients. The evoked responses were measured simultaneously by recording the transdiaphragmatic pressure with a differential pressure transducer and the adductor pollicis strength with a force displacement transducer. Cumulative vecuronium dose-response curves were determined for both muscles in 11 ASA class I adult patients. The mean (SD) doses required to depress adductor pollicis and diaphragm responses to 50% (ED50) were 30 (9) micrograms kg-1 and 37 (12) micrograms kg-1, respectively. Corresponding values for 95% depression of the twitch response (ED95) were 48 (13) micrograms kg-1 and 67 (23) micrograms kg-1 (P less than 0.02), indicating that the diaphragm required approximately 40% more vecuronium for subtotal abolition of the single twitch response.