Mitogen-activated protein kinase (MAPK) is activated in many cell types in response to growth factors during the G0-G1 transition in the cell cycle. We investigated the effects of platelet-derived growth factor (PDGF) AA and PDGF BB on activation of MAPK in human dermal fibroblasts, and asked whether its activation correlates with proliferative responses of human fibroblasts to PDGF AA and PDGF BB. Treatment with either PDGF isoform for 20 min resulted in equal phosphorylation of MAPK as visualized by gel shifts in Western blotting with anti-MAPK polyclonal antibody. This finding was confirmed by measurements of MAPK activity in response to increasing doses (2-20 ng/ml) of PDGF AA and PDGF BB in in vitro assays with myelin basic protein as a substrate. PDGF AA was slightly less potent than PDGF BB, but both growth factors induced similar maximal activations. Kinetics of activation were also similar for both isoforms, with maximal induction of MAPK at 10-20 min after growth factor addition followed by a gradual decline to control levels at 1 h. Activation of MAPK by both PDGF isoforms was also confirmed by measuring myelin basis protein phosphorylation in MAPK immunoprecipitates. Thus both PDGF AA and PDGF BB are potent activators of MAPK in human dermal fibroblasts. In contrast, PDGF BB elicited a strong mitogenic response, while PDGF AA had no significant effect on DNA synthesis in human dermal fibroblasts. These data indicate that acute activation of MAPK is not sufficient to stimulate cells to progress through cell cycle. PDGF AA may have other biologic functions or may play a co-stimulatory role in proliferation of human dermal fibroblasts.