FceRI-activation-induced survival of mast cells is dependent on the expression and function of the prosurvival protein A1. The expression of A1 in lymphocytes and monocytes has previously been described to be transcriptionally regulated by NF-k-B. Here we demonstrate that the expression of A1 in mast cells is not dependent on NF-kB but that NFAT plays a crucial role. FceRI-induced A1 expression was not affected in mast cells overexpressing an IkB-u super-repressor or cells lacking NF-kB subunits RelA, c-Rel, or c-Rel plus NF-K-B1 p50. In contrast, inhibition of calcineurin and NFAT by cyclosporin A abrogated the expression of A1 in mast cells on FctRI-activation but had no effect on lipopolysaccharide-induced expression of A1 in J774A.1 monocytic cells. Cyclosporin A also inhibited luciferase expression in an A1 promoter reporter assay. A putative NFAT binding site in the A1 promoter showed inducible protein binding after FceRI crosslinking or treatment with ionomycin as detected in a band shift assay or chromatin immunoprecipitation. The binding protein was identified as NFAT1. Finally, mast cells expressing constitutively active NFAT1 exhibit increased expression of A1 after FceRI-stimulation. These results indicate that, in FceRI stimulated mast cells, A1 is transcriptionally regulated by NFAT1 but not by NF-kB.