The authors determined the onset time and the duration of action of neuromuscular blockade by a 100-150 micrograms.kg-1 vecuronium random administration in 30 ASA I and ASA II patients during urological surgery. Neuromuscular blockade was evaluated by the electromyographic response to stimulation of the ulnar nerve train of four. The time from vecuronium administration to complete abolition of twitch response (T1 = 0%) did not change significantly, in spite of 50% increased dose of neuromuscular relaxant. The duration of action of neuromuscular blockade, that is the time from T1 = 0% to T1 = 25%, increased from 38.2 +/- 7.5 min to 52.5 +/- 19.8 min by the increasing of neuromuscular relaxant dose (p < or = 0.018). Vecuronium increased doses did not shorten significantly the onset time.