Bilateral complete transection of the fimbria-fornix causes a significant increase in nerve growth factor (NGF) content both in the septum and hippocampus of the adult rat as measured by a sensitive immunoassay 7 days after lesioning. The finding, that elevation of NGF in septum (250%) was much more pronounced than that in the hippocampus (30-50%), cannot be explained by retrograde axonal transport. Degeneration of central cholinergic neurons might be a potential trigger for NGF production in the CNS.