Succinylcholine chloride is the most commonly used muscle relaxant. Its rapid onset of action and relatively brief duration are unique Despite its wide use, certain pharmacologic aspects of auccinylcholine chloride are not as widely appreciated as they should be. There is marked variation in the responses of patients to clinically used doses. Large doses demonstrate that recovery from succinylcholine chloride is slower than is generally appreciated in man. The dose related neuromuscular blocking effect of succinylcholine chloride in cats was investigated using a cat common peroneal nerve anterior tibial muscle preparation. All experimental cats tracheas were intubated through a tracheostomy under general anesthesia with Nembutal 40 mg/kg intravenously. Respiration was controlled by a Harvard animal respirator. The body temperature was kept at 35~37 ℃ by a thermoblanket. The degree of neuromuscular block following intravenous succinylcholine chloride, 0.5 mg/kg and 1 mg/kg, were measured by single twitch response. The common peroneal nerve was stimulated supramaximally by a single stimulus with square waves, 0. 2 msec duration and at a frequency of 0.1 Hz. The ratio of the twitch height was calculated. The results were as follows: 1) The time of neuromuscular blokade to 100% depression was 30.7 sec and to l00% spontaneous recovery was 1,260 sec (21 min.) in the succinylcholine chloride 0.5 mg/kg intravenous group. The recovery index was 258. 5 sec (4. 3 min.). 2) The time of neuromuscular blockade to 100% depression was 30 sec and to 100% spontaneous recovery was 2,004 sec (33. 4 min.) in the succinylcholine chloride I mg/kg intravenous group. No significant time difference was observed in neuromuscular depression in both groups but spontaneous recovery time was markedly prolonged to 744 sec (59% prolongation). The recovery index was also prolonged to 474 sec (83% prolongation).