The cholinergic structures in the forebrain of the rat were studied immunohistochemically with a monoclonal antibody to choline acetyltransferase (ChAT) which has proven to be particularly suited to reveal the trajectory of ChAT-positive fibers and their terminations. The habenulo-interpeduncular tract contained the most distinctly ChAT-positive fibers. Here, ChAT-positivity was demonstrated in the perikarya, fibers and terminal field. From the basal forebrain, ChAT-positive fiber bundles could be followed rostrally to the olfactory bulb, dorsomedially to the neocortex and hippocampus, and dorsocaudally to the hippocampus. Terminations of ChAT-positive fibers in these regions were either boutons terminaux (in the interpeduncular nucleus), finely granular (in the hippocampus) or a network of varicose fibers (in the olfactory bulb and neocortex). Cholinergic terminal fields were also demonstrated in the following regions: the nucleus of the lateral olfactory tract, the olfactory tubercle, the pyriform cortex, the lateral nucleus and part of the basal nucleus of the amygdala, the caudate-putamen, the nucleus accumbens, some thalamic nuclei, and the suprachiasmatic nucleus of the hypothalamus. In most of these regions, ChAT-positivity appeared as either fine granules or a meshwork of varicose fibers.