Denervation injuries in baboons were used to define the time course of the hyperkalemic response to succinylcholine. Half-peak increase in serum potassium (2.7S mEq/1) occurred 8.4 days following injury. Peak increase (5.5 mEq/1) appeared 14 days after injury. However, changes in potassium levels begin as early as four days after injury. Succinylcholine or other depolarizing muscle relaxants should not be used after the fourth day following an Injury or denervation that involves two or more limbs.