Stress stimuli such as free radicals, high osmolarity or arsenite activate stress-activated protein kinases (SAPKs) in a wide variety of cells. In the present study, we have investigated the ability of several stress stimuli to activate SAPKs in platelets and to induce phosphorylation of their substrates. Treatment of human platelets with H₂O₂ stimulated SAPK2a and its downstream target mitogen-activated protein kinase-activated protein kinase-2 (MAPKAP-K2). Kinase activity reached a maximum after 2-5 min and declined towards basal levels after 15 min. Arsenite caused a steady increase of MAPKAP-K2 activity up to 15 min. The level of maximal kinase activation by H₂O₂ and arsenite was comparable with the effect caused by the physiological platelet stimulus thrombin. A high osmolarity solution of sorbitol induced comparatively small activation of SAPK2a and MAPKAP-K2. The 42-kDa extracellular signal-regulated kinase (ERK) 2 was not activated by H₂O₂, sorbitol or arsenite. None of these stimuli triggered significant arachidonic acid release on their own. However, H₂O₂ and sorbitol enhanced the release of arachidonic acid induced by the calcium ionophore A23187. This effect was reversed by the inhibitor of SAPK2a, 4-(4-fluorophenyl)-2-(4-methylsulphinylphenyl)-5-(4-pyridyl) imidazole (SB 203580), but not by the inhibitor of the ERK2-activating pathway, 2-(2-amino-3-methoxyphenyl)-oxanaphthalen-4-one (PD 98059). Both H₂O₂ and sorbitol increased phosphorylation of cytosolic phospholipase A₂ (cPLA₂) and its intrinsic activity; both responses were blocked by SB 203580. Phosphorylation of cPLA₂ by H₂O₂ occurred on Ser-505, a reaction that is known to increase the intrinsic lipase activity of the enzyme. Our results demonstrate that activation of SAPKs by stress stimuli primes cPLA₂ activation through phosphorylation. In vivo, this mechanism would lead to the sensitization of platelet activation and may be an important risk factor in thrombotic disease.