Microinjection of mRNA from adult rat brains into Xenopus oocytes induced an electrophysiological response to acetylcholine in the cells. The cells responded to acetylcholine through muscarinic receptors which appeared on the cell surface after microinjection of the mRNA and showed the depolarization, mainly due to an increase in membrane permeability to Cl ions. The acetylcholine reaction did not require extracellular Ca ions and was suppressed by pertussis toxin, suggesting the involvement of the inhibitory guanine nucleotide-binding regulatory protein.