Brain-derived neurotrophic factor (BDNF) has therapeutic potential for treatment of the injured central nervous system. BDNF induces both differentiation and survival of neurons by binding to trkB receptors. This interaction stimulates the intrinsic tyrosine kinase activity of trkB, initiating a signal cascade involving the phosphorylation of intracellular protein on tyrosine, serine, and threonine residues. The purpose of this investigation was to examine the effects of cationic lipid-mediated gene transfection of BDNF on phosphatidylinositol 3 (PI3)-kinase activity in primary septo-hippocampal cell cultures. Thirty-six hours after BDNF gene transfection in the primary CNS cell culture, PI3-kinase activity was significantly increased. The increased PI3-kinase activity was inhibited by wortmannin, a selective and irreversible inhibitor of PI3-kinase. In addition, wortmannin blocked neurofilament increases induced by BDNF gene transfection. This result suggests a possible role of PI3-kinase activation in neuroprotective effects produced by BDNF gene transfection.