The effect of intravenous injection of ketamine 2, 5 and 10 mg.kg-1 on the neuromuscular blocking action of succinylcholine was studied on the indirectly stimulated adductor pollicis muscle twitch of monkeys anaesthetized with 0.5-1.0 per cent halothane in oxygen. Neuromuscular block was quantified by single twitches evoked at 0.1 Hz. The changing nature of neuromuscular block from Phase I to Phase II was monitored periodically by train-of-four fade. In the absence of succinylcholine, ketamine had no consistent neuromuscular effect of its own. In the presence of succinylcholine, ketamine in a dose-dependent manner potentiated both the Phase I and the Phase II neuromuscular blocking effect of succinylcholine. In Phase I, 2 mg.kg-1 of ketamine reduced the ED50 of succinylcholine from 0.46 +/- 0.07 mg.kg-1 to 0.33 +/- 0.06 mg.kg-1 (P less than 0.01), and increased its 25-75 per cent recovery index from 4.0 +/- 0.4 min to 5.3 +/- 0.1 min (P less than 0.01). In Phase II, ketamine in the same dose deepened a steady neuromuscular block maintained by succinylcholine infusion from 48 +/- 3 per cent block to 71 +/- 2 per cent block (P less than 0.01). We concluded that ketamine potentiates the Phase I and the Phase II neuromuscular blocks of succinylcholine.