The aim of this study was first to specify the morphology and neuronal environment of the large cholinergic neurons, and second to determine the distribution and mode of termination of the corticostriatal and dopaminergic inputs on these neurons in the rat striatum. Immunocytochemical procedures with a monoclonal antibody against choline acetyltransferase, Golgi staining and standard electron microscopic techniques were used to specify the ultrastructural features of the putatively cholinergic classical large neurons. The large/choline acetyltransferase-positive neurons are characterized by a voluminous, eccentric, and deeply indented nucleus leaving a large cytoplasmic area, and by the presence of an abundant granular endoplasmic reticulum and of many polysomes and free ribosomes. Serial ultrathin sectioning further indicated the presence of nematosomes or nucleolus-like bodies within the nucleus and the cytoplasm of the large neurons. In addition, these neurons were found to be in direct apposition with up to four surrounding neurons showing features typical of medium-sized spiny neurons. These data support the view that the putatively cholinergic neurons may have an intense metabolic activity and may be involved in striatal clusters. When choline acetyltransferase immunostaining was coupled with the identification of degenerating corticostriatal afferents after lesion of the cerebral cortex, degenerating terminals were seen to form synapses of an asymmetrical type on distal labelled dendrites, but these contacts were very rare. On the other hand, nigrostriatal dopaminergic axons, identified by means of either the degeneration method or tyrosine hydroxylase immunostaining, were often found to run directly for long distances around the choline acetyltransferase-positive cell bodies. Occasionally, dopaminergic terminals formed possible symmetrical synapses on choline acetyltransferase-positive cell bodies or proximal dendrites. These data provide evidence that the putatively cholinergic neurons are directly contacted by corticostriatal and dopaminergic nigrostriatal afferents. The respective positions and nature of the two types of contacts further provide morphological support for the hypothesis that postsynaptic interactions may occur between the corticostriatal and dopaminergic nigrostriatal afferents at the level of the cholinergic neurons.