There is accumulating evidence that the p38 Map kinase pathway plays important roles in Type I interferon (IFN) signaling, but the mechanisms regulating p38 activation during engagement of the Type I IFN receptor remain to be defined. We sought to identify the events that lead to activation of the p38 Map kinase in response to Type I IFNs. Our data demonstrate that treatment of sensitive cell lines with IFN-alfa results in activation of both Map kinase kinase 3 (Mkk3) and Map kinase kinase 6 (Mkk6). Such IFN-inducible activation of Mkk3 and Mkk6 is essential for downstream phosphorylation and activation of the p38 Map kinase, as shown by studies using mouse embryonic fibroblasts (MEFs) with targeted disruption of the Mkk3 and Mkk6 genes (Mkk3-/-Mkk6-/-). Similarly, IFN-dependent activation of the downstream effectors of p38, MapKapK-2 and MapKapK-3, is not detectable in cells lacking Mkk3 and Mkk6, demonstrating that the function of these Map kinase kinases is required for full activation of the p38-pathway. To define the functional relevance of Mkk3/6 engagement in Type I IFN signaling, IFN-inducible gene transcription was evaluated in the Mkk3/Mkk6 double knockout cells. IFN alfa- and IFN beta-dependent transcription via either ISRE or GAS elements was defective in Mkk3 -/-Mkk6 -/- MEFs in luciferase reporter assays. In addition, IFN-dependent induction of two genes known to be of importance in the generation of IFN-responses, ISG15 and IRF-9, was diminished in the absence of Mkk3 and Mkk6. The effects of Mkk3 and Mkk6 on IFN-dependent transcription were unrelated to any effects on the phosphorylation and activation of Stat-proteins, indicating the presence of a Stat-independent mechanism. Altogether, our findings demonstrate that Mkk3 and Mkk6 are rapidly activated during engagement of the Type I IFN receptor and play important roles in Type I IFN-signaling and the generation of IFN-responses.