Both prolactin (PRL) and TGF-β regulate cell survival in mammary epithelial cells, but their mechanisms of interactions are not known. In primary mammary epithelial cells and the HC11 mouse mammary epithelial cell line, PRL prevented TGF-β-induced apoptosis, as measured by terminal deoxynucleotidyltransferase dUTP nick-end labeling staining and caspase-3 activation. This effect depended on phosphatidyl inositol triphosphate kinase (PI3K). PI3K activates a downstream serine/ threonine kinase, Akt; therefore, we investigated the role of Akt in the interaction between PRL and TGF-β signaling. Akt activity was inhibited by TGF-β over a 20- to 60-min time course. In TGF-β-treated cells, PRL disinhibited Akt in a PI3K-dependent manner. Expression of dominant negative Akt blocked the protective effect of PRL in TGF-β-induced apoptosis. Transgenic mice overexpressing a dominant-negative TGF-β type II receptor (DNIIR) in the mammary epithelium undergo hyperplastic alveolar development, and this effect was PRL dependent. Involution in response to teat sealing was slowed by overexpression of DNIIR; furthermore, Akt and forkhead phosphorylation increased in the sealed mammary glands of DNIIR mice. Thus, Akt appears to be an essential component of the interaction between PRL and TGF-β signaling in mammary epithelial cells both in vitro and in vivo.