Platelet-derived growth factor (PDGF) isoforms lead to mitogenic, survival, and chemotactic responses in a variety of mesenchymal cell types during development and in the adult. We have studied the importance of phosphatidylinositol-3′ kinase (PI3K) signaling in these responses by mutating the PI3K-binding sites in the PDGF-β receptor by gene targeting in embryonic stem cells. Homozygous mutant mice developed normally; however, cells derived from the mutants were less chemotactic and had largely lost their ability to contract collagen gels in response to PDGF. Injection of a mast cell degranulating agent in mice led to a decrease in interstitial fluid pressure resulting in edema formation. In contrast to wild-type mice, mutant mice were unable to normalize the pressure after treatment with PDGF. Taken together, these observations suggest a function for PDGF signaling through PI3K in interstitial fluid homeostasis by modulating the tension between cells and extracellular matrix structures.