The role of intracellular Ca 2+ pools in the regulation of growth factor signal transduction pathways and mitogenesis is not well understood. We have examined the roles of basal and transiently mobilized Ca 2+ in the regulation of MAP kinases by EGF. To assess the influence of Ca 2+ transients we utilized Plcg1−/− and Plcg1+/+ mouse embryonic fibroblasts, while BAPTA/AM was employed to chelate total intracellular Ca 2+ in the same cell lines. The MAP kinases erk-1, erk-2 and erk-5 exhibited similar patterns of activation in wild-type and Plcg1−/− cells treated with EGF. However, pretreatment with BAPTA/AM significantly increased and prolonged erk-1 and erk-2 activation in both cell types. In contrast, BAPTA/AM prevented the EGF activation of erk-5 in wild-type and Plcg1−/− cells. These data indicate that basal Ca 2+, but not growth factor provoked Ca 2+ transients, has a significant influence on the activation of these MAP kinases. AG1478, a specific EGF receptor kinase inhibitor, abolished the prolonged erk-1 and erk-2 activation produced by EGF in cells pretreated with BAPTA/AM. This indicates that the prolonged activation of erk-1 and erk-2 produced in the presence of BAPTA/AM requires continuous signaling from the EGF receptor kinase.