Hyperosmotic shock induces cytochrome c release and caspase-3 activation in Xenopus oocytes. Different signaling pathways engaged by osmostress converge on the mitochondria to trigger cell death. The mitogen-activated protein kinases (MAPKs) JNK1-1 and JNK1-2 are early activated by hyperosmotic shock and sustained activation of both isoforms accelerates the apoptotic program. Indeed, sustained activation of p38 accelerates osmostress-induced cell death, but the p38 isoforms involved are not well characterized. Here we study the expression and activation of Xenopus p38 isoforms in response to hyperosmotic stress. We find that p38α, p38β, and p38γ are early activated by hyperosmotic shock and sustained activation of p38α and p38β accelerates osmostress-induced apoptosis. Moreover, microinjection of cytochrome c in the oocytes induces caspase-3 activation and p38α and p38β phosphorylation suggesting that caspases and kinases are interlinked in a positive feedback loop to promote cell death. In summary, we present a more complete view of the mechanisms involved in osmostress-induced apoptosis.