The cholinergic innervation of the rat basolateral amygdaloid nucleus (BL) was determined by the immunocytochemical localization of the acetylcholine biosynthetic enzyme, choline acetyltransferase (ChAT). ChAT-immunoreactive (ChAT-IR) elements were observed throughout the BL in the form of fine puncta and varicose fibers. Electron microscopy revealed that the immunoreactive puncta represented small terminals (0.3-1.2 micron), most of which formed synaptic contacts with unlabeled dendritic shafts or spines. Less frequently, ChAT-IR terminals established synaptic contacts with large neuronal cell bodies, which had all the characteristics of projection neurons as defined on the basis of axonal projections to the ventral striatum. ChAT-IR terminals were sometimes seen to form synaptic contacts with small neuronal cell bodies, including those of ChAT-IR neurons. The ChAT-IR boutons contained pleomorphic clear vesicles of varying size, and the large majority of the synapses were of the symmetric type. Small ChAT-IR neurons were observed in all parts of the BL. Although the ChAT-IR cell bodies varied widely in shape from typical fusiform to round, most had a more or less oval shape with a major diameter of 10-14 micron. Most of the ChAT-IR neurons seemed to display a radial bipolar dendritic pattern, but multipolar cells were also observed. The ChAT-IR neurons contained an indented nucleus, which was often eccentrically located and surrounded by a thin or moderately thin rim of cytoplasm. The results obtained are discussed in relation to a quasi-cortical organization of the BL.