This prospective study evaluated the hypothesis that emergence agitation after electroconvulsive therapy (ECT) could be caused by lactate-induced panic secondary to insufficient neuromuscular blockade. Plasma lactate levels were measured before and after 245 consecutive ECT sessions in 37 patients monitored for evidence of post-ECT agitation. ECT was administered using a brief-pulse, rectangular, constant-current device through bilaterally placed electrodes under general anesthesia and neuromuscular blockade. Agitation was observed in 7% of all ECT sessions. No significant difference could be found in pre-ECT lactate levels. However, mean post-ECT lactate levels in agitated sessions were significantly greater than those in nonagitated sessions (4.77 versus 2.54 mmol/l, p < 0.05). An increase (+27%) in the pre-ECT succinylcholine dose for those patients who previously had repeated post-ECT agitation resulted in cessation of post-ECT agitation and return of the formerly high post-ECT lactate levels to normal (1.61 versus 2.07 mmol/l). Although the number of patients who had post-ECT agitation was small, the data support the hypothesis that post-ECT agitation might be a manifestation of lactate-induced panic.