Transcriptional cyclin-dependent kinases 8 and 19 (CDK8 and CDK19) are enzymatic components of the Mediator complex. CDK19 is presumably a minor paralog of CDK8 and is thought to compensate for lack of CDK8 in certain cases, but evidence for unique functions of the two kinases remains sporadic. Experiments with tumor cell lines have yielded the vast majority of information on the role and mechanisms of action of the kinases, and the results might be irrelevant because of the changes accumulated by tumor cells. Mice with an inducible Cdk8 knockout and a constitutive Cdk19 knockout (Cdk8 ^fl/fl/Cdk19^-/-/ Rosa26/Cre/ER^T2) were therefore engineered for the first time to study the joint role of CDK8 and CDK19 in the while organism and primary cell cultures. The mice were used to obtain Cdk19^-/- mouse embryonic fibroblasts (MEFs) in which a Cdk8 knockout could be induced with 4-hydroxytamoxifen (4-OHT). Unlike most tumor cells, MEFs were sensitive to CDK8/19 inhibition and knockout: the inhibition led to significant cell death within 5 days, while the knockout decreased the cell proliferation rate. RNA sequencing revealed alterations in the Wnt signaling pathway, cytokine response, and osteoclast differentiation in Cdk8/Cdk19 knockout MEFs. Expression of genes associated with steroidogenesis was lower, consistent with earlier data. Changes associated with the cytoskeleton, adipogenic differentiation, osteogenic differentiation, cell adhesion, extracellular matrix formation, and mitochondrial biogenesis were described for the first time. Studies of the stress responses in MEFs showed that responses to DNA damage from X-ray irradiation and to serum stimulation after starvation were also mediated by CDK8/19 and were significantly reduced in Cdk8/Cdk19 knockout cells.