Cells in the nucleus basalis--diagonal band complex provide a largely cholinergic projection to all parts of the telencephalon, including the olfactory system, amygdala, hippocampus and neocortex. Experiments using fluorescent dyes as double retrograde axonal tracers indicate that although the projection as a whole is very widespread, the projection of individual cells is limited to a very restricted area within the cortex, not more than 1-1.5 mm in diameter.