Around the time of birth, cardiac muscle cells lose the capacity to divide and, from this time on, growth of the heart occurs by hypertrophy where each cells gets bigger. The hypertrophic response is characterized by changes in gene expression including expression of the atrial natriuretic factor (ANF) and myosin light chain-2 (MLC-2) genes. In cultured neonatal ventricular myocytes, hypertrophy also involves reorganization of contractile proteins into sarcomeric units. We have investigated the role of the Raf-1 kinase in this response. Activation of an estradiol-regulated Raf-1 protein kinase led to activation of mitogen-activated protein (MAP) kinase and activated expression from the ANF and MLC-2 promoters. Raf-1-induced activation of these genes was inhibited by a kinase deficient mutant of the 44-kDa MAP kinase, Erk1 indicating a requirement for MAP kinases in the Raf-1-induced response. However, activation of Raf-1 was not sufficient to induce the organization of actin into sarcomeric units. Transfection of dominant negative Raf-1 inhibited phenylephrine-induced activation of the ANF and MLC-2 promoters. Transactivation was rescued by the introduction of increased amounts of c-Raf suggesting a role for Raf-1 in the response to alpha-adrenergic agonists. These results suggest that activation of Raf-1 kinase is a critical component of the signal transduction pathway leading to changes in gene expression associated with hypertrophy but that Raf-1 is not sufficient for the regulation of actin organization during the hypertrophic response.