In the present study we employed immunohistochemical techniques using a polyclonal antibody against choline acetyltransferase (ChAT) to determine the distribution and cytological features of cholinergic neurons in the developing septal/diagonal band complex of the rat. ChAT-positive perikarya were first clearly detected in this region on embryonic day 17, although the neurons were faintly labeled and lacked the cytological details found in the adult. After birth we observed a dramatic increase in the intensity of the immunolabeling which continued until postnatal day 23. Thereafter, the ChAT-positive neurons assumed their adult-like characteristics.