PDGF heterodimer of A and B chains, a complete mitogen for 3T3 mouse fibroblasts, exemplifies those growth factors interacting with membrane associated tyrosine kinase receptors. Its binding to the PDGF-receptors results in receptor dimerization and subsequent activation of tyrosine kinase activity in the cytoplasmic protein domain, autophosphorylation of the receptor being the first event in the transduction cascade. Before the ligand-receptor complex is internalized and degraded, receptor stimulation is transmitted to the general transduction network, in which several tyrosine kinase substrates are activated by phosphorylation and changes the cytoplasmic biochemistry. These changes include cytoplasmic alkalinization, increases in the intracellular concentration of cyclic-AMP and Ca2+ and activation of protein kinase C through the degradation of phosphoinositides. The known substrates recruited by the PDGF-receptor association are phosphatidylinositol-3'-kinase, ras-GTPase-activating protein, phospholipase C-gamma, serine-threonine kinase Raf-1 and src and src-related tyrosine kinases. Upon binding of PDGF to its receptor, transactivation of transcriptional and nuclear factors such as c-fos and c-myc genes and dephosphorylation of c-jun occurs, V-sis, the oncogen of the simian sarcoma virus (SSV), is highly homologous to the c-sis/PDGF-B gene that encodes the homodimer of the B-chain of the PDGF receptor. Cells transformed by SSV have been studied as a model system for the autocrine stimulation of the PDGF receptor.