The activation of the mitogen-activated protein kinase cascade is one of the major signalling pathways by which growth factors transmit their mitogenic messages from the cell membrane to the nucleus. Two major breakthroughs reported in the past months are the cross-communication between the mitogen-activated protein kinase cascade and the cAMP-protein kinase A signal pathway, and the role of α- and βγ-complexes of heterotrimeric G proteins in activating the mitogen-activated protein kinase pathway. These signalling strategies have now also been demonstrated in renal mesangial cells. Another important step has been the identification of a candidate gene for polycystic kidney disease. Knowledge of the molecular mechanisms of action of growth factors in the kidney will promote greatly our understanding of the aetiology of renal disease.