We have studied the ability of ERK1,2, JNK1,2 and p38 kinases to be regulated after serum deprivation in E1A + E1B-19 kDa- and E1A + E1A + c-Ha-ras-transformed rat embryo fibroblasts. It was demonstrated that oncogene transformation resulted in an increase of total kinase content independently of the type of complementing oncogene. However, for ERK1,2 kinases phosphorylation was found to depend on the type of complementing oncogene. Besides, unusual biphasic character for ERK1,2 kinases phosphorylation was checked in control fibroblasts REF52 and in transformed E1A + E1B-19 kDa cells, which undergo G1/S arrest after a 24 h serum starvation. According to the immunoblotting data, phosphorylated forms of ERK1,2 kinases are not detected after 15-30 min of serum deprivation, but their content is restored up to the control level within several hours. At the same time, the level of ERK1,2 phosphorylation in E1A + c-Ha-ras cells did not change after serum withdrawal. Besides, serum deprivation did not lead to significant changes in the level of phosphorylation of both type stress kinases--JNK2 and p38 in all types of studied cells. We discuss possible mechanisms of biphasic alteration in ERK1,2 phosphorylation level under condition of serum deprivation of REF52 cells and E1A + E1B-19 kDa-transformed fibroblasts, able to be arrested in G1 phase.