The effect of intravenous succinylcholine on scrum potassium was studied in 21 combat casualties receiving their first anesthesia within three hours of sustaining massive muscle trauma. In 20 of these patients, there were no statistically or physiologically significant increases in serum potassium. In one patient, an increase in serum potassium was apparent ten minutes after injection of succinylcholine, progressing to a peak level of 6.2 mEq/1, 20 minutes after injection. It is uncertain whether succinylcholine caused the hyperkalemia in this patient.