The signaling pathways underling the transition of cardiomyocyte growth from hyperplasia in fetal/newborn to hypertrophy in postnatal/adult hearts are not well understood. We have shown that -adrenergic receptor (AR)-mediated regulation of neonatal cardiomyocyte proliferation involves p70S6K. In the current study, we examined the ontogeny of PI3K/p70S6K signaling pathway in rat heart and investigated the influence of AR on this pathway during development. Cardiac PI3K and p70S6K1 activities were high in E20 fetus, decreased gradually postnatally and were low in adult. In contrast, p70S6K2 was barely detectable. Phosphorylation of p70S6K1, Akt and PDK1 were markedly increased in late gestation and early postnatal life but not in adult heart. PTEN, a negative regulator of PI3K, was highly expressed in adult, but only at low levels and mostly in the phosphorylated (inactivated) form in fetus. AR stimulation resulted in increased cardiac p70S6K1 activity only in animals 2-week old and beyond, while Akt level was increased in all developmental stages tested. These increases were accompanied by an increase in BAD (Ser-136) phosphorylation without changes in PTEN level. Thus, there is a globally high input of cardiac PI3K signaling during the fetal-neonatal transition period. Inactivation of PTEN may in part contribute to the high activity of PI3K signaling, which coincides with the period of high cardiomyocyte proliferation. AR stimulation activates cardiac p70S6K1 and Akt in postnatal animals, and may activate cardiac survival signals. These data provide further evidence for the importance of AR and PI3K signaling in regulation of cardiac growth during development.