There are many reports that d-tubocurarine has marked species and individual variations in the matter of muscle relaxation. Therefore the dose-related neuromuscular blocking effect of d-tubocurarine was studied in anesthetized patients. Eighty adult patients were studied during halothane-N2O-O2 anesthesia for various kinds of surgery. These patients were in good physical shape, without known or suspected liver, kidney, neuromuscular or hormonal disease, and were not taking medication known to influence the action of relaxants. Preanesthetic medication consisted of atropine 0.01mg/kg and valium 0.18 mg/kg intramuscularly 60 minutes before anesthesia. Anesthesia was induced with thiopental 4~5mg/kg. Succinylcholine 1mg/kg was used to faciliate tracheal intubation. Moderate hyperventilation was maintained throughout by a mechanical ventilator and patient temperature was maintained at 35~36 ℃ during the study. Forty patients received a single intravenous d-tubocurarine 0.1mg/kg(group A) after the succinylcholine effect had worn off and the other forty patients received a single intravenous d-tubocurarine 0.2mg/kg(group B) after the succinylcholine effect had disappered. Prior to the administration of d-tubocurarine, the patient's forearm and hand were firmly fixed to a specially constructed metal armboard. The thumb was abducted, fixed and connected to a electrical kymograph, Harvard apparatus. The ulnar nerve was stimulated at the wrist subcutaneous needle electrodes by means of supramaximal stimull from a peripheral nerve stimulator(model 91-M3, Emerson, USA). Square wave of "Train of Four" stimuli of 0.2msec duration were delivered at a frequency of 0.1Hz. The evoked twitch and tracing of the adductor muscle of the thumb was recorded continuously on a electrical kymograph. Upon recovery from the initial dose of succinylcholine, a stable baseline twitch tension was recorded for 15 minuts. At this point a single intravenous injection of d-tubocurarine, either 0.1mg/kg or 0.2mg/kg, was given. The data were plotted as onset time of twitch depression and percentage depression of twitch height and the recovery time of "Train of four" response were analysed in each group. The results were as follows: 1) In d-tubocurarine 0.1mg/kg group: the effect of d-tubocurarine was variable, ranging from no effect on twitch tension to abolition of the twitch response. Mean maximal twitch depression was 59.4%. Recovery index which showed more than 75% twitch depression was 39.9minutes. 2) In d-tubocurarine 0.2mg/kg group: the effect of d-tubocurarine obtained consistent and solid neuromuscular relaxation. Mean maximal twitch depression was 95.2%. Recovery index was 58.1 minutes.