The p38 Mitogen activated protein Kinases (MAPKs) are a family of stress-activated proteins, normally activated in stress conditions, such as immune response and inflammation. They induce cell cycle arrest, apoptosis, differentiation and cell proliferation. The four known genes of the p38 family share ~60% sequence identity among themselves and ~45% with the members of the other three MAPK families. Since the same MKKs activate the different p38 MAPK, we aim to define the differences and the specificity in the signaling cascades induced by the different p38 MAPKs. To approach this goal we employed large-scale proteomics and phospho-proteomics technologies using p38β/α (-/-) mouse embryonic fibroblasts (MEFs) expressing HA-tagged wild-type p38α or p38β, the intrinsically active mutants of these p38β/α,or an empty vector. This way the effect of each variant could be elucidated independently from other extracellular stimulation and upstream activation. The three cell populations of p38α and p38β were labeled with light, heavy and medium stable isotope amino acids in tissue culture (SILAC) and mixed together. The tryptic peptides and enriched phosphopeptides were analyzed by capillary LC-MS/MS and thousands of phospho-sites and proteins were identified. Several changes in the mutant/w.t ratio of the proteins or the phosphopeptides were observed. Surprisingly, instead of the expected elevation in the phospho-sites known to be associated with the MAPK signaling pathway, we observed that these phospho-sites did not change much in their levels and a reduction in the levels of some of these sites were observed. For example,the phosphorylated state of p38α was down-regulated following the expression of the constitutively active p38β.We assume that an adaptation mechanism takes place due to the absence of of specific MAPK and due to the addition of a constitutively active one.