In 24 ASA I-II adults anaesthetized with thiopentone, fentanyl and nitrous oxide in oxygen, we studied neuromuscular transmission with isometric adductor pollicis monitoring. Patients received mivacurium 0.2 mg kg-1 followed by an infusion lasting at least 60 min and adjusted to maintain twitch height at 1-5%. After termination of the mivacurium infusion, when twitch height spontaneously regained 25% of its control value, the patients were allocated to two groups of 12 patients each. In group NEO patients received neostigmine 40 micrograms kg-1 and atropine 15 micrograms kg-1 and in group SPO neuromuscular transmission was allowed to recover spontaneously. Twitch height was measured every 10 s and train-of-four (TOF) (2 Hz) every 3 min. After 15 min, residual force after tetanic stimulation (50 and 100 Hz, 5-s duration (RF50HZ, RF100HZ), 1 min apart) were recorded sequentially. At 15 min, mean TOF ratio was greater in group NEO (0.94 (SEM 0.01)) than in group SPO (0.87 (0.02)) (P < 0.01). All patients in group NEO recovered to a TOF ratio greater than 0.7 after 6 min compared with 15 min in group SPO (P < 0.005). A TOF ratio greater than 0.9 was observed in all patients in group NEO compared with only six in group SPO (P < 0.025). Nevertheless, RF50HZ and RF100HZ did not differ significantly (0.92 (0.01) (group NEO), 0.91 (0.01) (group SPO) and 0.83 (0.02) (group NEO), 0.78 (0.03) (group SPO), respectively).(ABSTRACT TRUNCATED AT 250 WORDS)