Using a standardized anaesthetic protocol, the continuous monitoring of the twitch height after a 0.1 Hz stimulus was used to follow the evolution of curarization following injection of either atracurium (0.6 mg . kg-1) or alcuronium (0.2 mg . kg-1). The maximum twitch height inhibition was always greater than 99% of the control value and occurred after 107 +/- 50 s with atracurium (n = 30) and 172 +/- 120 s for alcuronium (n = 30) (p less than 0.02). Although surgical stage of muscular relaxation (twitch height less than 25% of reference value) was the same for both drugs (55 +/- 15 min for alcuronium versus 52 +/- 10 min for atracurium; n = 30 for both groups), the clinical duration (spontaneous restoration of twitch height to 90% of the reference value) was significantly longer (p less than 0.005) for alcuronium: 89 +/- 20 min (n = 10) versus 62 +/- 9 min for atracurium (n = 10). The spontaneous return to normal of the train of four was also significantly longer (p less than 0.001) for alcuronium: 118 +/- 23 min (n = 10) versus 69 +/- 7 min for atracurium (n = 10). The recovery index (the time required for twitch height to rise from 25% to 75%) was three times quicker (p less than 0.01) for atracurium (10 +/- 3 min; n = 10) than for alcuronium (30 +/- 13 min; n = 10).(ABSTRACT TRUNCATED AT 250 WORDS)