Glial cell line-derived neurotrophic factor (GDNF) was infused continuously for 2 weeks into the ventricles of male Wistar rats that had received a unilateral knife transection of the fimbria/fornix. In vehicle-treated, control animals, there was a 70% loss of choline acetyltransferase (ChAT)-positive and a 60% loss of p75-positive neurons in the septum/diagonal band ipsilateral to the axotomy as identified by immunohistochemistry, with no loss in ChAT biochemical activity. GDNF treatment at 10 micrograms/day completely prevented the loss of p75-positive neurons, significantly reduced the loss of ChAT-positive neurons to 40% of normal, and stimulated ChAT biochemical activity to 40% more than normal in an axotomy-dependent manner. GDNF is 1 order of magnitude less potent than NGF but, unlike NGF, had little or no effect on normal, uninjured neurons. GDNF was 1 order of magnitude more potent than BDNF, and BDNF had no effect on ChAT biochemical activity. GDNF and NGF inhibited weight gain, whereas BDNF induced significant weight loss and death at the dosage of 100 micrograms/day.