Levels of tyrosine hydroxylase (TH) and the ultrastructural relation between axons from cerebral cortex and TH containing, predominantly dopaminergic terminals were examined in the adult rat neostriatum at 2 and 12 days following unilateral decortication. The caudate nuclei from the unlesioned and lesioned hemispheres were biochemically assayed for TH processed for light or electron microscopic localization of the enzyme. At both time intervals examined, there was no statistically significant alteration in TH activity or apparent change in the intensity of reactive labeling visualized by light microscopy. However, electron microscopic examination of the caudate nucleus homolateral to the decortication at two days following surgery revealed the presence of numerous small, osmiophilic boutons which were much less frequently seen on the contralateral side. Further ultrastructural examination showed that the osmiophilic boutons formed predominantly asymmetric, axodendritic synapses. In sections containing both degenerating and TH labeled terminals, two patterns of connectivity could be discovered. First and most commonly, the degenerating and TH-labeled terminals formed synapses with the same dendrite or dendritic spine. Less frequently, the two types of terminals were in direct contact with each other. In this axo-axonic relation, the outer membranes between the terminals were in apposition but usually failed to exhibit pre- or postsynaptic specializations. These findings indicate that the cortical and dopaminergic nigral efferents have actions on common recipient neurons in the rat caudate nucleus and provide support for a possible direct axonal interrelationship between these two primary inputs.