The intracellular messenger mitogen-activated protein kinase (MAPK) is activated in vascular smooth muscle cells (SMC) by various growth factors as well as by agonists that have no proliferative effect. We explored the hypotheses that SMC proliferation is associated with a specific pattern of MAPK activation by evaluating the kinetics of MAPK activation and tyrosine phosphorylation and the intracellular location of MAPK in SMC following addition of agonists of varying mitogenic potential. A peak in MAPK activation and tyrosine phosphorylation occurred 3-10 min after the addition of agonists to SMC derived from human saphenous vein (early phase), followed by a plateau of activity, which was variable in duration (late phase). A correlation was not found between mitogenicity and the degree to which MAPK became activated or tyrosine phosphorylated in the early phase. However, the duration of MAPK activation and tyrosine phosphorylation correlated strongly with the ability of agonists to stimulate SMC proliferation. Nuclear translocation of MAPK was associated with SMC proliferation, although the degree to which each agonist induced nuclear translocation did not parallel its mitogenic potential. The relative dependency of all three events on protein kinase C differed for each agonist and was greater in the late versus the early phase. Thus, in human SMC, nuclear translocation of MAPK and prolonged activation and tyrosine phosphorylation of MAPK are associated with growth factor-induced mitogenesis.