Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor which has been purified on the basis of its ability to promote the survival of dopaminergic neurons in vitro. GDNF has subsequently been cloned and its sequence shown to be distantly related to transforming growth factor-β (TGF-β). To identify GDNF expressing cells in the adult rat brain, in situ hybridization using a digoxygenin (DIG)-labelled riboprobe has been performed. Our results show that GDNF mRNA is mainly expressed in neurons and that its synthesis is not restricted to dopaminergic areas. It is widely expressed in the cortex, the hippocampus, the striatum, the substantia nigra, the thalamus, the cerebellum and the spinal cord. Neuronal GDNF expression varies among brain regions as determined by the intensity of the in situ signal. Double labelling of the substantia nigra using tyrosine hydroxylase immunohistochemistry, associated with GDNF in situ hybridization, show that the majority of dopaminergic neurons express GDNF. The widespread expression of GDNF throughout the adult brain suggests that its administration in Parkinson's disease should be restricted to the altered structures, in order to avoid possible deleterious side effects.