The TrkB receptor protein-tyrosine kinase is a receptor for brain-derived neurotrophic factor and neurotrophin-3. In response to brain-derived neurotrophic factor and neurotrophin-3 treatment, TrkB expressed exogenously in Rat-2 cells is rapidly phosphorylated on tyrosine residues. At least 2 regions of TrkB contain phosphorylated tyrosines. The major sites of autophosphorylation are in the region containing Tyr-670, Tyr-674, and Tyr-675, which lies in the kinase domain and corresponds by sequence homology to the Tyr-416 autophosphorylation site in p60c-Src. Tyr-785, which lies just to the COOH-terminal side of the kinase domain in a relatively short tail characteristic of the Trk family of protein-tyrosine kinase receptors, is also phosphorylated in response to neurotrophin-3 treatment. The sequence around Tyr-785 fits a consensus sequence for binding phospholipase C-gamma 1. The simplest interpretation of these results is that, in response to neurotrophin binding, at least two and perhaps all three of the tyrosines in the Tyr-670/674/675 region are autophosphorylated independently, and Tyr-785 is autophosphorylated in vivo. Following activation of TrkB, phospholipase C-gamma 1 is phosphorylated on Tyr-783, Tyr-771, and Tyr-1254. Phospholipase C-gamma 1 also forms a complex with TrkB in response to neurotrophin-3 treatment, consistent with the possibility that one of the TrkB autophosphorylation sites provides a binding site for the phospholipase C-gamma 1 SH2 domains, as is the case for other receptor protein-tyrosine kinases. We conclude that phospholipase C-gamma 1 is directly phosphorylated by TrkB. Since phosphorylation of Tyr-783 and Tyr-1254 results in activation of phospholipase C-gamma 1, we predict that neurotrophin-3 leads to activation of phospholipase C-gamma 1 following binding to TrkB in Rat-2 cells.