The c-Myc oncoprotein regulates transcription of genes that are associated with cell growth, proliferation and apoptosis. c-Myc levels are modulated by ubiquitin/proteasome-mediated degradation. Proteasome inhibition leads to c-Myc accumulation within nucleoli, indicating that c-Myc might have a nucleolar function. Here we show that the proteins c-Myc and Max interact in nucleoli and are associated with ribosomal DNA. This association is increased upon activation of quiescent cells and is followed by recruitment of the Myc cofactor TRRAP, enhanced histone acetylation, recruitment of RNA polymerase I (Pol I), and activation of rDNA transcription. Using small interfering RNAs (siRNAs) against c-Myc and an inhibitor of Myc-Max interactions, we demonstrate that c-Myc is required for activating rDNA transcription in response to mitogenic signals. Furthermore, using the ligand-activated MycER (ER, oestrogen receptor) system, we show that c-Myc can activate Pol I transcription in the absence of Pol II transcription. These results suggest that c-Myc coordinates the activity of all three nuclear RNA polymerases, and thereby plays a key role in regulating ribosome biogenesis and cell growth