It has previously been shown that 30 min after administration of a single elec troconvulsive shock (ECS) to rats there is a marked decrease in both soluble and total acetylcholinesterase (AChE) activity in the midbrain and hippocampus. In the current study it has been shown that AChE activity is unchanged in these regions (apart from a small rise in soluble AChE in the hippocampus) 30 min after the final ECS of a series of 10 (once daily for 10 days). Twenty-four hours after the last ECS there was a significant increase in total and membrane bound AChE activity in the striatum but not hippocampus, midbrain, cortex and amygdala. This change in the striatum may be associated with the change in GABA synthesis which has been shown to also occur in this region at the same time.