Horseradish peroxidase injected into the rat globus pallidus was transported retrogradely to subthalamic nucleus neuronal cell bodies and anterogradely to axon terminals in the subthalamic nucleus. Electron microscopic observations revealed that the labeled axon terminals made symmetrical axosomatic and axo-dendritic synaptic contacts with labeled subthalamic nucleus perikarya and dendrites. Injection of kainic acid in the globus pallidus several days prior to the horseradish peroxidase injection abolished the anterograde but not the retrograde transport of the tracer. This suggested the anterograde labeling observed in the subthalamic nucleus originated from neuronal cell bodies in the globus pallidus. Kainic acid lesions identical to those employed in the above anatomical studies resulted in a loss of neuronal cell bodies throughout the globus pallidus and caused a drop in glutamic acid decarboxsylase and choline acetyltransferase levels in the globus pallidus. Levels of these two enzymes were not changed in the subthalamic nucleus after the globus pallidus kainic acid lesions, but both showed small, statistically significant decreases in the substantia nigra. It was concluded that there is a massive pathway from the globus pallidus to the subthalamic nucleus, which terminates on subthalamic nucleus neurons projecting back to the globus pallidus. Neither gamma-aminobutyric acid nor acetylcholine is the major neurotransmitter in the massive pallido-subthalamic pathway.