Interfering with cellular signal transduction pathways is a common strategy employed by viruses to create a propitious intracellular environment that fulfil the requirements for a whole and efficient virus multiplication cycle. Previous studies carried out by our group showed that the cellular infection by the vaccinia virus (VV), the prototype member of the Poxviridae family, genus orthopoxvirus, promotes a sustained activation of mitogen-activated proteins kinases (MAPKs) pathway, which leads to expression of the early growth response (egr-1) gene, and these events play a fundamental role for VV multiplication. The activation of at least two signaling pathways (MEK/ERK and MKK/JNK) and possibly, the participation of serine-threonine kinases (STKs) were demonstrated after VV infection. Thus, our aim is to investigate whether the STK p21-activated kinase 1 (PAK1), placed upstream of MAPKs cascade, would be the upstream connecting molecule upon VV infection. In this study, A31 murine fibroblasts were transfected with plasmids expressing the wild-type (WT) and dominant-negative (DN) forms of the protein PAK1. The transfected cells, selected with geneticin (G418), were serum-deprived for 12 hours and then infected at multiplicity of infection (MOI) 10. Cell lysates were then prepared to western blot and immunoblotted with anti-phospho JNK1/2, anti-phospho ERK1/2 and anti-Egr1 antibodies. Preliminary results have showed a strong reduction on JNK and ERK phosphorilation levels as well on Egr1 expression with the DN cells in comparison with WT cells, suggesting an effective participation of PAK1 during the VV infection. Therefore, this serine-threonine kinase, might be responsible for integrating the signals emanating from the cellular membrane to the downstream MAPKs cascade - MKK4/JNK and MEK/ERK/Egr1.