Muscarinic acetylcholine receptors (mAChR) in the embryonic chicken heart undergo agonist-induced internalization and a subsequent decrease in receptor number (downregulation). Cloning studies have identified two subtypes of mAChR expressed in the embryonic chicken heart, the cm2 and cm4 receptors. We report here that persistent activation of the mAChR in cultured chicken heart cells with the cholinergic agonist carbachol causes significant decreases in the levels of both cm2 and cm4 mRNA, as measured by solution hybridization analyses. The half-lives of the cm2 and cm4 mRNAs are not altered by agonist treatment, indicating that agonist most likely regulates mRNA levels by regulating the rate of gene transcription. Activation of mAChR in chicken heart causes both inhibition of adenylate cyclase activity and stimulation of phospholipase C activity. To test whether changes in the levels of intracellular second messengers were involved in the changes in mAChR mRNAs observed following agonist exposure, we determined the effects of incubation with agonists for the A1 adenosine receptors (which inhibit adenylate cyclase in chicken heart) and angiotensin II receptors (which stimulate phospholipase C) on mAChR receptor number and mRNA levels. Activation of these pathways together through heterologous receptors resulted in decreased mAChR number and mRNA levels, although these changes were not as large as those seen with direct activation of the mAChR. These results suggest that regulation of adenylate cyclase and phospholipase C activities may be involved in the regulation of mAChR gene expression.