Post-natal growth of cardiac muscle cells occurs by hypertrophy rather than division and is associated with changes in gene expression and muscle fiber morphology. We show here that the protein kinase MEKK1 can induce reporter gene expression from the atrial natriuretic factor (ANF) promoter, a genetic marker that is activated during in vivo hypertrophy. MEKK1 induced both stress-activated protein kinase (SAPK) and extracellular signal-regulated protein kinase (ERK) activity; however, while the SAPK cascade stimulated ANF expression, activation of the ERK cascade inhibited expression. C3 transferase, a specific inhibitor of the small GTPase Rho, also inhibited both MEKK-and phenylephrine-induced ANF expression, indicating an additional requirement for Rho-dependent signals. Microinjection or transfection of C3 transferase into the same cells did not disrupt actin muscle fiber morphology, indicating that Rho-dependent pathways do not regulate actin morphology in cardiac muscle cells. While active MEKK1 was a potent activator of hypertrophic gene expression, this kinase did not induce actin organization and prevented phenylephrine-induced organization. These data suggest that multiple signals control hypertrophic phenotypes. Positive and negative signals mediated by parallel MAP kinase cascades interact with Rho-dependent pathways to regulate hypertrophic gene expression while other signals induce muscle fiber morphology in cardiac muscle cells.