The interaction between succinylcholine (SCC) and non-depolarizers, atracurium or vecuronium was investigated in 36 cats of either sex using the sciatic nerve-anterior tibialis muscle preparation. Additionally, the relation of SCC to pseudocholinesterase activity was examined. The duration of action of vecuronium (6.5 ± 1.3 to 7.3 ± 2.2 minutes) in cats pretreated with SCC was greater than those (2.0 ± 0.6 minutes) in non-pretreated cats. However, SCC had no influence on the duration of atracurium. The serum pseudocholinesterase activity was decreased after the injection of atracurium or neostigmine in contrast to vecuronium. The authors conclude that the prior administration of SCC prolongs the duration of vecuronium but not that of atracurium, and pseudocholinesterase activity is not related to the prolonging effect of SCC.