BACKGROUND: Since neostigmine was introduced for reversal of neuromuscular block, there has been controversy about the optimum dose for antagonizing neuromuscular block. The purpose of this study was to characterise recovery of neuromuscular transmission following a vecuronium-induced block 15 min after neostigmine administration using different stimulation patterns, and to determine the effects of different doses of neostigmine given at various pre-reversal twitch heights. METHODS: Adductor pollicis (AP) mechanical activity in response to low (0.1 and 2 Hz) and high (50 and 100 Hz) frequency stimulation, was recorded 15 min after 20, 40 and 80 micrograms/kg neostigmine, given to reverse a vecuronium-induced block at 10, 25 and 50% pre-reversal twitch height (TH). Fifty four ASA class I and II anaesthetised (methohexital, fentanyl, N2O/O2) young adult patients were studied and randomly allocated into 9 groups of 6 patients each. RESULTS: In contrast to twitch height (TH) and residual force after 50 Hz, 5 s tetanic stimulation (RF50Hz), the greater sensitivity of train-of-four (TOF) ratio and residual force after 100 Hz, 5 s tetanic stimulation (RF100Hz) points out the best reversal conditions (prereversal TH and the optimal neostigmine dose) (P < 0.001, two-way analysis of variance). The highest reversal scores (about 0.9 TOF ratio and RF100Hz) were obtained when 40 micrograms/kg of neostigmine was given at 25 and 50% TH. A 0.9 TOF ratio was also observed when 40 micrograms/kg of neostigmine was given at 10% TH, but, under these conditions, RF100Hz was only 0.6 (P < 0.05, Duncan test). CONCLUSION: To optimise the reversal action of neostigmine in order to obtain the highest neuromuscular transmission recovery (0.9 TOF ratio and RF100Hz) during a vecuronium-induced neuromuscular block, the 40 micrograms/kg dose has to be given at 25 to 50% recovery of TH.