Apoptosis of cultured cerebellar granule neurons (CGNs) deprived of serum is prevented by K+ depolarization or moderate concentrations of N-methyl-d-aspartate (NMDA). Here, we have examined the role of the serine/threonine kinase Akt in these protective effects. The exposure of mouse CGNs to NMDA or K+ depolarization increased the phosphorylation of Akt, compared with that measured in cells incubated in a physiological K+ concentration. Only the NMDA-evoked response was reduced by inhibitors of phosphatidylinositol 3-kinase (wortmannin and LY294002) and mitogen-activated protein kinase (PD98059 and U0126). Similarly, the capacity of NMDA to inhibit apoptosis of CGNs deprived of serum was greatly reduced by these inhibitors as well as by the transfection of neurons with a catalytically inactive mutant of Akt, whereas the protective effect of K+ depolarization remained unaffected. These findings indicate that K+ depolarization and NMDA activate Akt through different signalling pathways in CGNs. Moreover, Akt mediates the anti-apoptotic effect of NMDA, but not that evoked by K+ depolarization.