alpha 1-Adrenergic agonists activate a hypertrophic response in cultured neonatal ventricular myocytes, which include an increase in cell size, organization of contractile proteins into sarcomeric units, and the induction of the atrial natriuretic factor (ANF) gene. Previous findings have supported a role for ras in this signaling pathway. Utilizing microinjection techniques to delivery affinity-purified neutralizing antibodies to G alpha q,11 into cultured ventricular myocytes, the current studies demonstrate a functional requirement for the heterotrimeric G protein, Gq, in the alpha 1-adrenergic induction of the ANF gene, changes in cell size, organization of myofilaments, and phosphoinositide hydrolysis. Expression of a constitutively active mutant of G alpha q leads to the expression of ANF protein in these cells. Taken together, these data suggest that G q-dependent pathways are necessary and sufficient to activate defined features of the hypertrophic response. In attempts to further delineate the relative roles of ras and Gq in this pathway, we found that G alpha q is required for alpha 1-adrenergic phosphoinositide hydrolysis, though ras does not appear to be necessary for this response. In addition, we coexpressed an inhibitory ras mutant, along with the constitutively active G alpha q. Expression of ANF protein stimulated by the G alpha q mutant was not inhibited. Thus, both ras- and Gq-dependent pathways are necessary to fully transduce defined features of alpha 1-adrenergic-stimulated hypertrophy of neonatal cardiac ventricular myocytes, but activated Gq may be able to induce ANF expression independent of inhibitory ras.