Cardiac hypertrophy involves the growth of heart muscle cells and is driven by faster protein synthesis which involves increased ribosome biogenesis. However, the signaling pathways that link hypertrophic stimuli to faster ribosome production remain to be identified. Here we have investigated the signaling pathways which promote ribosomal RNA synthesis in cardiomyocytes in response to hypertrophic stimulation. We employed a new non-radioactive labeling approach and show that the hypertrophic agent phenylephrine (PE) stimulates synthesis of 18S rRNA (made by RNA polymerase I) and 5S rRNA (produced by RNA polymerase III) in adult cardiomyocytes. In many settings, rRNA synthesis is driven by rapamycin-sensitive signaling through mammalian target of rapamycin complex 1 (mTORC1). However, the activation of rRNA synthesis by PE is not inhibited by rapamycin, indicating that its regulation involves other signaling pathways. PE stimulates MEK/ERK signaling in these cells. Inhibition of this pathway blocks the ability of PE to activate synthesis of 18S and 5S rRNA. Furthermore, BI-D1870, an inhibitor of the p90(RSK)s, protein kinases which are activated by ERK, blocks PE-activated rRNA synthesis, as did a second p90(RSK) inhibitor, SL0101. BI-D1870 also inhibits the PE-stimulated association of RNA polymerase I with the rRNA promoter. These findings show that signaling via MEK/ERK/p90(RSK), not mTORC1, drives rRNA synthesis in adult cardiomyocytes undergoing hypertrophy. This is important both for our understanding of the mechanisms that control ribosome production and, potentially, for the management of cardiac hypertrophy.