In co-cultures of rat septum and hippocampus, cholinergic neurons, identified by immunocytochemical techniques using antibodies against choline acetyltransferase, were found to be exclusively located in septal tissue. The presence of nerve growth factor during the entire growth period of four weeks increased the activities of acetylcholinesterase and choline acetyltransferase about 10-fold and strongly increased the number of acetylcholinesterase-positive neurons. Application of nerve growth factor yielded different effects depending on the age of the cultures. During the first two weeks in vitro, nerve growth factor enhanced the number of acetylcholinesterase-positive neurons, an effect which was no longer observed following later applications of nerve growth factor. Nerve growth factor increased the activities of cholinergic enzymes during all phases of in vitro development, but the effects of one-week applications were always considerably smaller than those observed following continuous application of nerve growth factor. The results of different application schedules suggest that the continuous presence of nerve growth factor is needed for maximal increases in cholinergic enzyme activities and maintenance of cholinergic neurons in septohippocampal co-cultures.