Following fimbria/fornix transection, cells in the medial septum appear to undergo retrograde degeneration as shown by Nissl and acetylcholine esterase (AChE) staining. Recent studies using immunocytochemical techniques have also demonstrated loss of choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr) labeling of neurons in this region. Whether the apparent loss of ChAT- and NGFr-positive neurons is the result of the actual death of these neurons, or is instead a loss of ChAT enzyme or NGFr expression below levels detectable by immunocytochemical methods, remains an unresolved issue. In order to address this question, rhodamine-labeled fluorescent latex microspheres were injected into the hippocampus where they retrogradely transported to the cell bodies of the medial septum. Five days later these animals received either unilateral or bilateral fimbria/fornix lesions and were allowed to survive an additional 4 weeks. Compared to unlesioned control animals, unilaterally lesioned animals showed a 91% loss of fluorescently labeled cells and bilaterally lesioned animals showed a 93% loss. The inability to detect the fluorescent microspheres in the medial septum suggests that the majority of medial septal cells die after fimbria/fornix transection. ChAT and NGFr immunohistochemical staining were also performed. Cells stained for ChAT were reduced in number by 92% in animals with unilateral lesions and by 75% in animals with bilateral lesions, while NGFr-stained cells were reduced in number by 75% in animals with unilateral lesions and by 68% in animals with bilateral lesions.(ABSTRACT TRUNCATED AT 250 WORDS)