We have previously shown that echovirus 1 (EV1) infection increases the mRNA levels of cellular immediate-early (IE) genes in host cells. Here we provide further evidence that the induction ofjunB,c-jun,and c-fosgenes is due to active viral macromolecular synthesis rather than to the interaction of EV1 with its receptor, α₂β₁integrin. Nuclear run-on transcription assays indicated that differences in mRNA levels in infected and uninfected cells are brought about by regulation at the transcriptional level. EV1 infection induced the phosphorylation of both the stress-related p38 mitogen-activated protein kinase (MAPK) and the growth signal-related ERK1/2 MAPKs. Studies with selective MAPK inhibitors revealed that p38 was the main inducer ofjunBexpression, whereas both MAPK pathways were involved in the induction of c-fos.Activation of AP-1 genes was also observed to occur during infections with other enteroviruses and with Semliki Forest A7(74) virus, suggesting that the phosphorylation of MAPKs and induction of AP-1 gene expression may be important regulators of host cell behavior during viral infections.