Background : The speed of action of nondepolarizing muscle relaxants might be correlated with the drug potency and the receptor-plasma concentration gradient. However, because of dissociation constants (KDs) being masked by plasma concentration following systemic administration, we investigated the degrees of train-of-four (TOF) fade at various stages of the measured first twitch height of TOF (T1) during recovery related to potency of various muscle relaxants using isolated forearm technique. Methods : Thirty two volunteers of the conscious healthy adults who were not receiving any medication which might have influence neuromuscular transmission were involved in this study. The electrodes were applied over the ulnar nerve at the wrist and supramaximal transcutaneous single twitch stimulation (0.1 Hz) delivered by a peripheral nerve stimulator (Innervator, Fisher & Paykel, New Zealand) using a 0.2 ms square wave pulses was applied. The twitch response of the thumb adductor was measured mechanomyographically using a 2 kg Load Cell strain gauge (Model No. 505H, RS Components Ltd., UK) with a thumb piece modification. Following a 5 min. period of stabilization, forearm pneumatic tourniquet was inflated to 300 mmHg in order to occlude the systemic circulation. The equipotent dose (ED95 0.1) of various muscle relaxants diluted in 20 ml of saline was injected into a vein on the dorsal hand respectively: rocuronium 30 g/kg, atracurium 25 g/kg, mivacurium 8 g/kg and vecuronium 4 g/kg. Eight volunteers received each dose. The forearm tourniquet was released at 3 min after drugs given. The neuromuscular block was allowed to recovery spontaneously, and at 25, 50, 75 and 100% recovery of control twitch height, TOF stimulation (2 Hz for 2 s) was administered in order to compare TOF ratios between musle relaxants. Results : In the aminosteroid compounds, the TOF ratios of rocuronium were greater than those of vecuronium at all of the assessment points during recovery. In the benzylisoquinolium compounds, the TOF ratios of atracurium were greater than those of mivacurium at 25 and 50% recovery of control twitch height. Conclusion : The degree of TOF fade during recovery is related to drug potency at presynaptic receptors separated from the effects of plasma drug concentration. (Korean J Anesthesiol 1998; 35: 684∼690)