The regulation of cellular signal transduction and growth by four human muscarinic acetylcholine receptor (mAChR) subtypes has been studied comparatively. The four mAChRs fall into two functional sub-groups, based on their primary effects on second messenger formation; two of the receptors strongly inhibit adenylyl cyclase activity, whereas the other two strongly stimulate PI hydrolysis. Studies on mAChR regulation of two cellular events involved in cellular growth regulation, the transcription of proto-oncogene c-fos and DNA synthesis, indicate that these events are efficiently activated by those mAChRs which couple primarily to phospholipase C.