Electrical stimulation of either the caudate nucleus, substantia nigra or lateral hypothalamus led to an increase in the concentration of acetylcholinesterase in cerebrospinal fluid collected from the cisterna magna of the rabbit. This effect was not due to gross cell damage, fluid loss or contamination with blood plasma. Cerebrospinal fluid collected from the lateral ventricle contained a lower concentration of acetylcholinesterase than that collected from the cisterna magna; furthermore, stimulation of the ipsilateral caudate nucleus caused no change in the concentration of this enzyme in ventricular cerebrospinal fluid. After placing a large lesion in the ipsilateral substantia nigra, the 'resting' concentration of acetylcholinesterase in cisternal cerebrospinal fluid was 43% of the value in unoperated rabbits and stimulation of the caudate nucleus no longer led to an increase in the concentration of acetylcholinesterase in cisternal cerebrospinal fluid. The possibility is discussed that when the caudate nucleus is stimulated, acetylcholinesterase is released from the substantia nigra.