Shortly after birth, cardiac myocytes lose the ability to divide, and, in adult animals, heart muscle grows by a process of cellular hypertrophy where each individual cell gets larger. We have previously shown that activated Ras protein can induce markers of the hypertrophic phenotype, including atrial natriuretic factor (ANF) expression and organization of contractile proteins, and that Ras is at least partially required for the hypertrophic effect of phenylephrine. In the present study, we examine the requirement for the mitogen-activated protein kinases (MAP kinases) in the hypertrophic response induced by phenylephrine. We find that phenylephrine treatment results in the activation of the MAP kinases and that this activity is required for transactivation of the fos, ANF, and MLH promoters. However, inhibition of MAP kinases does not prevent phenylephrine-induced organization of actin. These results suggest that the signal transduction pathways leading to different hypertrophic responses diverge upstream of the MAP kinases but possibly downstream of Ras.