The p38 mitogen-activated protein (Map) kinase plays a critical role in the generation of signals in response to stress stimuli, but its role in interferon (IFN) signaling and its potential regulatory role in the activation of Jak-signal transducer and activator of transcription (Stat) pathway are not known. In the present study, we provide evidence that the p38 Map kinase is rapidly phosphorylated and activated during treatment of cells with Type I interferons (IFNalpha and IFNbeta). Furthermore, the Type I IFN-dependent activation of p38 regulates induction of the catalytic domains of MapKap kinase-2 and MapKap kinase-3, strongly suggesting the existence of an IFNalpha signaling cascade activated downstream of the p38 kinase. The engagement of this pathway in interferon signaling plays a critical role in interferon-dependent transcriptional regulation, as evidenced by the fact that inhibition of p38 activation results in abrogation of interferon-dependent gene transcription via interferon-stimulated response elements. Interestingly, inhibition of the kinase activity of the p38 blocks IFNalpha-induced gene transcription without inhibiting DNA binding or tyrosine phosphorylation of Stat proteins, suggesting that the p38 pathway acts in cooperation with the Stat pathway. Thus, the p38 kinase signaling cascade is activated by the Type I interferon receptor and plays a critical role in interferon signaling and interferon-dependent transcriptional regulation.