Altered ceramide and sphingosine expression during the induction phase of ischemic acute renal failure. Recent evidence indicates that a “sphingomyelin signaling pathway” exists: in response to heterogeneous influences, sphingomyelin is hydrolyzed, liberating ceramide, and subsequently its sphingoid base, sphingosine. Ceramide and sphingosine can influence diverse cellular processes, including cell differentiation, proliferation, protein trafficking, and apoptosis. Each of these processes have important implications for post-ischemic acute renal failure (ARF). However, sphingosine and ceramide expression during the induction of ischemic/reperfusion injury have not been previously assessed. To this end, CD-I mice were subjected to 45 minutes of unilateral renal ischemia ± reperfusion, followed by cortical sphingosine, ceramide, and sphingomyelin assessments. Contralateral kidneys served as controls. Ischemia caused ∼50% sphingosine and ceramide decrements. During reperfusion, sphingosine rebounded to normal values. Conversely, ceramide rose to, and was maintained at, supranormal levels (∼175% of controls). Subsequent studies performed with hypoxic or oxygenated isolated proximal tubules suggested that these changes: (1) had a multifactorial basis; (2) were partially simulated by enhanced PLA₂ activity; (3) and were dissociated from alterations in net sphingomyelin content. To assess the potential pathogenic relevance of the documented ceramide increments, cultured human proximal tubule (HK-2) cells were subjected to ATP depletion/Ca²⁺ ionophore- or PLA₂-induced attack with or without exogenous C2 ceramide loading. Ceramide worsened both forms of injury without exerting an independent lethal effect. Conversely, ceramide markedly attenuated arachidonic acid cytotoxicity. This occurred without any decrease in arachidonate uptake, suggesting a direct cytoprotective effect. In conclusion: (1) sphingosine and ceramide fluxes are hallmarks of early ischemic/reperfusion injury; (2) these changes occur via divergent metabolic pathways; and (3) that ceramide increments can affect divergent injury pathways, and that sphingosine and ceramide have potent cell signaling effects, suggest that the currently documented sphingosine/ceramide fluxes could have important implications for the induction phase and evolution of post-ischemic ARF.