THE mitogen-activated protein kinase (MAPK) signalling cascade is a prominent cellular pathway used by many growth factors, hormones and neurotransmitters to regulate physiological responses(1,2). Although activation of the MAPK pathway by receptors with tyrosine kinase activity is well defined(3), the mechanism used by heterotrimeric G-protein-coupled receptors to activate this pathway is less clear. Here we show that in cells deficient in the Src-related tyrosine kinase Lyn, stimulation of MAPK kinase and MAPK by G(q)-coupled mi muscarinic acetylcholine receptors (mAChR) is blocked, whereas G(i)-coupled m2 mAChR-mediated stimulation is unaffected. In cells deficient in the tyrosine kinase SyK, both m(1) and m2 mAChRs failed to stimulate MAPK kinase and MAPK. This result indicates that Syk is essential for the G(i)-coupled pathway and that Lyn and Syk are necessary for the G(q)-coupled pathway.