The synaptic relationships between leucine-enkephalin containing axon terminals and cholinergic neurons in the rat globus pallidus were studied at both light and electron microscopic levels using a high resolution pre-embedding double-labeling immunocytochemical method. Results indicated that leucine-enkephalin terminals very rarely form monosynaptic connections with cholinergic neurons in the rat globus pallidus, suggesting that enkephalinergic neostriatal efferents probably have little monosynaptic influences on the activities of pallidal cholinergic neurons.