The effects of chronic agonist exposure on receptor number (down-regulation) have been shown, in part, to be due to effects on mRNA levels. Agonist-mediated effects on muscarinic acetylcholine receptor (mAChR) mRNA were investigated in Chinese hamster ovary (CHO) cells stably transfected with m1 mAChR gene constructs containing the open reading frame and a series of deletions of the flanking 3'-untranslated region (3'-UTR). Carbachol (CBC) down-regulated m1 mAChRs encoded by the construct m1C1, an m1 mAChR transcript containing the entire flanking 3'UTR (nucleotides 1526-2622), in a time-dependent fashion with maximal decreases occurring by 12 h. Steady-state levels of m1C1 mRNA declined in a parallel fashion beginning 6 h after CBC pretreatment. Similar findings were obtained with m1C2, a construct which is missing all but 261 bases of flanking 3'-UTR (nucleotides (nt) 1526-1786). Since the rate of mRNA degradation represents an important potential regulatory mechanism to control the level of gene expression, we investigated the effects of CBC treatment on m1C1 and m1C2 mRNA stability. The half-life of either transcript in untreated cells was approximately 14 h, whereas m1C1 and m1C2 transcript half-lives decreased to approximately 3 h in cells treated with CBC. Agonist-induced destabilization of m1C2 mRNA could be mimicked by phorbol esters in a concentration-dependent manner and blocked by the protein kinase inhibitor, H-7. In contrast, m1 mAChR mRNA constructs missing nt 1526-1786 of the 3'-UTR (m1C3 and m1C4) did not undergo agonist- or phorbol ester-induced destabilization. In the neuroblastoma cell line IMR-32, endogenous m1 mAChR mRNA was down-regulated and destabilized following CBC treatment. These results demonstrate that agonist-induced mRNA destabilization is a potential mechanism for regulating m1 mAChR levels. Furthermore, deletion studies identify a 261 base region of the 3'-UTR having the potential to form stable stem-loop structures which likely harbors element(s) responsible for message destabilization.