Immunocytochemistry with a monoclonal antibody against choline acetyltransferase has been used to characterise cholinergic neurons in the rat neostriatum. The light microscopic morphology, ultrastructure and synaptic input of these neurons was compared to that of the three types of large neuron found in Golgi preparations of the striatum. The cholinergic neurons are large and have long infrequently branching dendrites. Two of the immunoreactive neurons were also Golgi-impregnated and showed characteristics of the "classical" large neurons of the striatum. Examination in the electron microscope revealed that the synaptic input to perikarya and proximal dendrites is sparse, thus distinguishing them from another large type of neuron, found in the ventral regions of the striatum, whose dendrites and perikarya are ensheathed in synaptic boutons. It is concluded that one of the three morphologically distinguishable classes of large neuron in the striatum is a cholinergic neuron.