Cimetidine increases the duration of action of succinylcholine several-fold by an unknown mechanism. The hydrolysis rate of succinylcholine by human plasma was measured with a modified spectrophotometric assay. At a concentration of 1-50 micrograms/ml cimetidine did not inhibit the hydrolysis of succinylcholine. It is concluded that cimetidine may have an effect at the neuromuscular junction but does not inhibit plasma cholinesterase.