Seven-transmembrane receptor (7TMR) signaling is transduced by second messengers such as diacylglycerol (DAG) generated in response to the heterotrimeric guanine nucleotide–binding protein G q and is terminated by receptor desensitization and degradation of the second messengers. We show that β-arrestins coordinate both processes for the G q -coupled M1 muscarinic receptor. β-Arrestins physically interact with diacylglycerol kinases (DGKs), enzymes that degrade DAG. Moreover, β-arrestins are essential for conversion of DAG to phosphatidic acid after agonist stimulation, and this activity requires recruitment of the β-arrestin–DGK complex to activated 7TMRs. The dual function of β-arrestins, limiting production of diacylglycerol (by receptor desensitization) while enhancing its rate of degradation, is analogous to their ability to recruit adenosine 3′,5′-monophosphate phosphodiesterases to G s -coupled β 2 -adrenergic receptors. Thus, β-arrestins can serve similar regulatory functions for disparate classes of 7TMRs through structurally dissimilar enzymes that degrade chemically distinct second messengers.