We have recently shown that the presence of albumin in astrocytes triggers the synthesis and release of oleic acid, which behaves as a neurotrophic factor for neurons. Thus, oleic acid promotes axonal growth, neuronal clustering, and the expression of the axonal growth-associated protein, GAP-43. In this work we show that oleic acid upregulates GAP-43 expression by a protein kinase C (PKC)-dependent mechanism. Since GAP-43 expression has been shown to be upregulated by several neurotrophins, we investigated the relationship between the effect of oleic acid and that of NGF, neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5) on GAP-43 expression. Our results indicate that NGF is not involved in the neurotrophic effect of oleic acid because the addition of NGF did not modify the effect of oleic acid on GAP-43 expression. Neither NT-3 nor NT-4/5 alone modified GAP-43 expression. However, NT-3 and NT-4/5 acted synergistically with oleic acid to increase GAP-43 expression. The lack of effect of NGF as compared to other neurotrophins is not unexpected since we have not found TrkA expression under our experimental conditions. The effect of oleic acid on GAP-43 expression must be independent of autocrine factors synthesized by neurons because this effect was also observed at low cellular densities. In conclusion, our results indicate that oleic acid behaves as a neurotrophic factor, inducing GAP-43 expression through a PKC-mediated mechanism that is not mediated by other neurotrophic factors but that is strongly synergized by NT-3 and NT-4/5.