Following single intravenous injections of succinylcholine chloride in doses of 0.5 to 4.0 mg/kg, the duration of muscle paralysis to 50% recovery lasted from 5.9 to 17.1 min; duration was dose dependent. Following intramuscular injection of succinylcholine chloride, 1.0 to 4.0 mg/kg, the latent period was between 1.8 and 2.7 min, and dependent on both dose and the addition of hyaluronidase. The duration of weakness was quite variable, with 50% recovery requiring between 20 and 27 min. There was a strong suggestion that the formation of Phase II nondepolarizing block is not only dependent on the dose of succinylcholine chloride, but upon the duration of depolarization of the myoneural junction.