Signaling in cell proliferation, cell migration and apoptosis is highly affected by osmotic stress and changes in cell volume, although the mechanisms underlying the significance of cell volume as a signal in cell growth and death are poorly understood. In this study, we used NIH3T3 fibroblasts in a serum- free inorganic medium (300 mOsm) in order to analyze the effects of osmotic stress on the MAP kinase activity and on the PDGFRmediated signal transduction. We found that hypoosmolarity (cell swelling at 211 mOsm) induced the phosphorylation and nuclear translocation of Erk1/2 apparently via a pathway independent of PDGFRand Mek1/2. Conversely, hyperosmolarity (cell shrinkage at 582mOsm) moved nuclear and phosphorylated Erk1/2 to the cytoplasm and induced phosphorylation and nuclear translocation of p38 and phosphorylation of Jnk1/2. In a series of parallel experiments, hypoosmolarity did not affect PDGF-BB induced activation of PDGFRwhereas hyperosmolarity strongly inhibited ligand-dependent PDGFRactivation as well as downstream mitogenic signal components of the receptor, including Akt and the Mek1/2-Erk1/2-pathway. Based on these results we conclude that ligand dependent activation of PDGFRand its downstream effectors Akt, MEK1/2 and Erk 1/2 is strongly inhibited by hyperosmotic cell shrinkage whereas cell swelling does not seem to affect the activation of the receptor but rather to activate Erk1/2 via a different mechanism. It is thus likely that cell swelling via activation of the MAPKinase Erk and cell shrinkage via activation of the p38 and JNK pathway and inhibition of the PDGFR-signaling pathway may act as key players in regulation of tissue homeostasis.