We here investigate the crosstalk of PKC and PKA signaling during primary CD3(+) T-lymphocyte activation using pharmacologic inhibitors and activators in combination with our established panel of PKC isotype-deficient mouse T cells in vitro. PKCtheta and PKA inversely affect the CD3/CD28-induced IL-2 expression, whereas other PKC isotypes are dispensable in this signaling pathway. Gene ablation of PKCtheta selectively results in a profound reduction of IL-2 production; however, complete abrogation of IL-2 production in these PKCtheta(-/-) T cells was achieved only by simultaneous coactivation of the cAMP/PKA pathway in CD3(+) T cells. Conversely, the reduced IL-2 production in PKC inhibitor-treated T cells can be rescued by inhibition of the cAMP/PKA pathway in wild-type but not in PKCtheta(-/-) T cells. Mechanistically, the cAMP/PKA and PKCtheta pathways converge at the level of NF-AT, as shown by DNA binding analysis. The combined increase in PKA and decrease in PKCtheta activity leads to an enhanced inhibition of nuclear NF-AT translocation. This PKCtheta/PKA crosstalk significantly affects neither the NF-kappaB, the AP-1, nor the CREB pathways. Taken together, this opposite effect between the positive PKCtheta and the negative cAMP/PKA signaling pathways appears rate limiting for NF-AT transactivation and IL-2 secretion responses of CD3(+) T lymphocytes.