Cardiac hypertrophy and dilatation can result from stimulation of signal transduction pathways mediated by heterotrimeric G proteins, especially G_q, whose α subunit activates phospholipase Cβ (PLCβ). We now report that transient, modest expression of a hemagglutinin (HA) epitope-tagged, constitutively active mutant of the G_q α subunit (HAα^*_q) in hearts of transgenic mice is sufficient to induce cardiac hypertrophy and dilatation that continue to progress after the initiating stimulus becomes undetectable. At 2 weeks, HAα^*_q protein is expressed at less than 50% of endogenous α_q/11, and the transgenic hearts are essentially normal morphologically. Although HAα^*_q protein declines at 4 weeks and is undetectable by 10 weeks, the animals develop cardiac hypertrophy and dilatation and die between 8 and 30 weeks in heart failure. As the pathology develops, endogenous α_q/11 rises (2.9-fold in atria; 1.8-fold in ventricles). At 2 weeks, basal PLC activity is increased 9- to 10-fold in atria but not ventricles. By 10 weeks, it is elevated in both, presumably because of the rise in endogenous α_q/11. We conclude that the pathological changes initiated by early, transient HAα^*_q expression are maintained in part by compensatory changes in signal transduction and other pathways. Cyclosporin A (CsA) prevents hypertrophy caused by activation of calcineurin [Molkentin, J. D., Lu, J.-R., Antos, C. L., Markham, B., Richardson, J., Robbins, J., Grant, S. R. & Olson, E. N. (1998) Cell 93, 215–228]. Because HAα^*_q acts upstream of calcineurin, we hypothesized that HAα^*_q might initiate additional pathways leading to hypertrophy and dilatation. Treating HAα^*_q mice with CsA diminished some, but not all, aspects of the hypertrophic phenotype, suggesting that multiple pathways are involved.