The MAP kinases Erk1/2 are the major mediators of the oncogenicity of the Receptor tyrosine kinase (RTK)-Ras-Raf-MEK cascade. As components of this pathway are often mutated and constantly active in cancer, Erks are regarded as promising targets for therapy for many types of malignancies. Yet, while active Erks are promoting abnormal cell proliferation, they simultaneously also inhibit upstream components of the pathway. Erks’ inhibition causes, therefore, reactivation of proto-oncogenic proteins hence Erks’ inhibitors must be applied causiously. Here we studied the effects of Erks’ inhibitors on cells transformed by oncogenic variants of Erk1, Erk1R84H and Erk1R84S, in which the negative feedback effect is so powerful that Erks’ phosphorylation could not be monitored. We show that provision of either BVD523, SCH772984, ASN007, Temuterkib or GCD0994, led to re-appearance of high phosphorylation levels of Erks, suggesting that all inhibitors intercepted Erks-mediated negative feedback activity. We found that inhibitors-mediated Erks’ re-phosphorylation is dependent entirely on MEK activation and that the highly phosphorylated Erk molecules are catalytically inactive. All inhibitors caused cell cycle arrest at the G1 phase, by inducing elevation of p21Cip1/Waf1 and Klf4, but did not affect cell viability for at least 72 hours. They also caused activation of the MAP/stress kinase p38. Proteome and phosphoproteome analysis of BVD523-treated cells revealed effect of the drug on the Rho GTPase pathway and on MAPK targets. The 5 inhibitors were further tested on a battery of 9 cancer-derived cell lines and in many of them most of the effects were similar to those observed with the Erk1R84H- and Erk1R84S-transformed NIH3T3 cells