The importance of the cell cycle in proliferating cells is well known, but little is known about the role of cell cycle regulatory proteins in cardiac myocytes, which are fully differentiated cells. The present study determined, in vitro, the effect of angiotensin II (Ang II) treatment of neonatal rat cardiac myocytes on protein levels of cyclins and retinoblastoma gene product (pRb) phosphorylation. The role of G₁ cyclin/cdk in Ang II-induced cardiac myocyte hypertrophy by overexpressing cdk inhibitor p21^Cip1/Waf1 or p16^INK4a was also examined using recombinant adenoviral vectors encoding these genes. Western blot analysis revealed that Ang II stimulated cyclin D1, D2, D3 and A protein levels in cardiac myocytes. Moreover, Ang II phosphorylated pRb on serine 780, which is known to occur in mitotic cells during cell cycle progression. Cultured cardiac myocytes treated with Ang II and infected with either control or recombinant adenovirus indicated that expression of p21 and p16 inhibited Ang II-induced cardiac myocyte hypertrophy, [³H]leucine incorporation into total cellular proteins, and skeletal α-actin (SK-A) and atrial natriuretic peptide (ANP) mRNA accumulation. Control virus had no effects on these parameters. These results suggest that G₁ cyclins play an important role in cardiac myocyte hypertrophy stimulated by Ang II.