Phosphoinositide 3-kinases (PI3Ks) have been grouped into three major classes that have different substrate specificities. Class IA PI3Ks consist of a catalytic and a regulatory subunit and have multiple isoforms that arise from different subunit combinations. The role of two of the small regulatory subunits, p55alpha and p50alpha, is poorly understood. We have now identified a novel function for these subunits and have shown that their expression is specifically induced in the involuting mouse mammary gland where they are involved in the downregulation of PI3K signalling and Akt/PKB activity. This abrogation of survival signalling thru Akt/PKB and its downstream targets is essential for the induction of apoptosis. The switch from lactation to involution is associated with activation of the transcription factor Stat3, by the cytokine LIF. Stat3 is essential for the induction of apoptosis and, in the absence of Stat3 or LIF, expression of the p55alpha and p50alpha subunits is abrogated. Surprisingly, Stat3 is a direct regulator of p55alpha and p50alpha expression, as demonstrated using ChIP assays, and therefore these subunits are not splicing isoforms as previously thought. An important implication of our results is that the p55alpha and p50alpha small regulatory subunits are regulated independently of the larger p85alpha subunit, and have an essential role in Stat3-mediated apoptosis in mammary gland.