Phosphorylation and activation of cytosolic phospholipase A2 (PLA2) can occur independently of the activation of 42/44-kDa mitogen-activated protein (MAP) kinase in human platelets. We have investigated the hypothesis that the stress-activated p38 MAP kinase plays a role in the regulation of cytosolic PLA2 The specific inhibitor of p38 MAP kinase, SB 203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl) imidazole], completely blocked the collagen-stimulated phosphorylation of cytosolic PLA2 in the presence of a cyclooxygenase blocker, and reduced the release of [3H]arachidonic acid by low concentrations of collagen. Stimulation of platelets with collagen (100 pg/ml) enhanced in vitro PLA2 activity of platelet lysates twofold over basal levels. in vitro PLA2 activity was reduced to basal levels when platelets were stimulated in the presence of SB 203580, but not in the presence of an inhibitor of the kinase that activates p42/p44 MAP kinase. SB 203580 only partially inhibited phosphorylation of cytosolic PLA2 in platelets that had not been treated with a cyclooxygenase blocker indicating that secondary stimulation by thromboxane A, induces cytosolic PLA, phosphorylation, by kinase(s) other than p38 MAP kinase. Under these conditions, inhibition of p42/p44 MAP kinase did not result in a reduction of cytosolic PLA, phosphorylation, which is in agreement with the results obtained in the presence of cyclooxygenase blockers. In contrast to collagen, both p38 MAP kinase and p42/p44 MAP kinase participated in the phosphorylation of cytosolic PLA, in platelets stimulated by cross-linking of the low-affinity receptor for immune complexes, FcyRIIA. The present results demonstrate an important role for p38 MAP kinase in the regulation of cytosolic PLA2 activity in collagen-stimulated human platelets.