CHO cells express both of the phosphoinositidase C-linked G-proteins Gq and G11 G11α is some 2.5-fold more highly expressed than Gqα in membranes of these cells. Following transfection and stable expression of CHO cells with DNA encoding the human muscarinic M1 acetylcholine (HM1) receptor, chronic treatment of the cells with the cholinergic agonist carbachol resulted in down-regulation of membrane levels of both Gqα and G11α. Dose-response curves to carbachol produced identical EC50 values for agonist-induced down-regulation of the two G-proteins and both were down-regulated with the same time course. These data indicate that the HM1 receptor interacts with and activates both Gqα and G11α equivalently and non-selectively in a whole cell system in which the receptor has access to both G-proteins.