The sudden onset of respiratory acidosis or alkalosis due to inadequate ventilation during general inhalation anesthesia may influence the action of neuromuscular blocking agents. In virtro animal studies by Funk et al.(1980) suggested that the neuromuscular blocking action of Org NC 45(NC) was minimally depressed above pH 7.68 and significantly potentiated under acidotic conditions(pH 7.05). They proposed that this was the result of an increase in NC metabolism by alkaline hydrolysis in the alkalotic state and greater molecular stability during acidosis. This study was performed to determine the effects of the neuromuscular blocking action of NC during respiratory acidosis and alkalosis. The patients were divided in to 3 groups: 1, ll & lll and experienced normocarbia, hypocarbia and hypercarbia, respectively. Hypocarbia was induced by hyperventilation and hypercarbia by adjustment of a rebreathing valve in the CO2 absorber in the semiclosed system. Simultaneously, arterial blood samples were collected from radial arteries for arterial blood gas analysis including pH and pCO2. Following the administration of succinylcholine(SCC) and the recovery of a 75% twitch height, ED95 of NC was given to the patient and the results were recorded by an evoked electromyograph (NMT, Datex). The results are follows: 1) The number of patients in groups l, ll and lll were 22, 13 and 8, respectively. The patients in each group were evenly distributed with respect to age, body weight and anesthesia. 2) The end-tidal CO2 tension in group l, ll and lll group was 38.86+/-4.62, 20.23+/-2.42 and 52.00+/-4.86mmHg, and the arterial pCO2(pH) was 37.36+/-5.71(7.461+/-0.054), 23.00+/-1.51(7.649+/-0.032) and 53.29+3.35 mmHg(7.314+/-0.026), resptectively. The end-tidal CO2 tension, arterial CO2 tension and pH in group ll and lll were significantly different from those of group l(p<0.05). 3) The onset time of SCC in group ll and lll was shorter than that in group l (p<0.05), but within 1 min in all groups. The duration of SCC in group lll(19.56+/-6.15min) was longer than that in group l (14.74+/-4.56min) (p<0.05). 4) Although there was no significant difference among the groups with respect to onset time and duration, the recovery index in group ll(10.29+/-2.21min) was significantly different from group l and lll(14.76+/-5.26 and 13.50+/-13.67 min, respectively) (p<0.05). After administration of NC in 5 min intervals, twitch tension was measured and the results were inserted into a regression equation which emphasized the delayed recovery in group lll(r=0.87). In conclusion, the recovery index in alkalosis shortened and the initial twitch tension in acidosis following NC administration was delayed compared to that in normocapnis and alkalosis. Patients with alkalosis may require more frequent doses of NC and continuous monitoring following repeated or continuous infusion in acidosis.