Significant differences in whole brain and in brain area acetylcholine concentration have been observed between the two strains of rats selectively bred for differences in speed of acquisition of conditioned avoidance. Brain acetylcholinesterase activity was similar in the strains. The strain showing poor conditioned avoidance behaviour possessed the highest brain concentration of acetylcholine. The finding supports a theory which describes a central cholinergic inhibitory system which may be active in the control of certain types of learning behaviour.