Recent evidence indicates that glial cell line-derived neurotrophic factor (GDNF) may influence microglial survival, proliferation, and activation, but this has not yet been tested on isolated primary microglia. We compared the effects of individual and combined application of 10 ng/ml GDNF and 1 ng/ml transforming growth factor-beta1 (TGF-beta1) on total cell number, 5-bromo-2'-deoxyuridine (BrdU) incorporation, DNA nick-end labelling (TUNEL staining), and nitrite and lactate dehydrogenase (LDH) secretion in serum-free cultures of primary rat microglia. GDNF as well as TGF-beta1 enhanced the total number of lectin-positive cells and decreased the number of TUNEL-positive nuclei, while no effect on proliferation was observed. Both factors suppressed the secretion of nitrite during the first 4 days of culturing, and GDNF but not TGF-beta1 reduced the secretion of LDH in 2-week-old cultures. These findings suggest that GDNF and TGF-beta1 support survival of primary microglia in vitro.