We recently showed that the transient expression of a hemagglutinin (HA) epitope–tagged, constitutively active mutant of the G protein &agr;qsubunit (HA&agr;q*) in the hearts of transgenic mice is sufficient to induce cardiac hypertrophy and dilatation that continue to progress after HA&agr;q* protein becomes undetectable. We demonstrated that the activity of phospholipase C&bgr;, the immediate downstream target of activated G&agr;q, is increased at 2 weeks, when HA&agr;q* is expressed, but also at 10 weeks, when HA&agr;q* is no longer detectable. This observation suggested that the transient HA&agr;q* expression causes multiple, persistent changes in cellular signaling pathways. We now demonstrate changes in the level, activity, or both of several signaling components, including changes in the amount and hormone responsiveness of phospholipase C&bgr; enzymes, in the basal level of diacylglycerol (which predominantly reflects activation of phospholipase D), in the amount or distribution of protein kinase C (PKC) isoforms (PKC&agr;, PKC&dgr;, and PKC&egr;), and in the amount of several endogenous G proteins. These changes vary depending on the isoform of the signaling molecule, the chamber in which it is expressed, and the presence or absence of HA&agr;q*. Our results suggest that a network of linked signaling functions determines the development of hypertrophy. They also suggest that atria and ventricles represent different signaling domains. It is likely that such changes occur in other model systems in which the activity of a single signaling component is increased, either due to an activating mutation or due to overexpression of the wild type.