12(S)-HpETE is a monohydroperoxy polyunsaturated fatty acid (PUFA) produced by the action of platelet or leukocyte 12-lipoxygenase (12-LO) on arachidonic acid. It activates human blood leukocyte 5-LO, resulting in the synthesis of 5(S)-HETE, leukotriene B4 (LTB4), and 5(S),12(S)-DiHETE. Rat lung metabolizes 12(S)-HpETE to 8,11,12- and 10,11,12-trihydroxyeicostrienoic acids. 12(S)-HpETE is the mediator of many biological functions, including induction of c-fos and c-jun, activation of AP-1, and endothelium-dependent vasoconstriction. It mediates the inhibitory synaptic response to FMRF-amide in Aplysia sensory neurons and inhibits Ca2+/calmodulin-dependent protein kinase II from rat brain cortex.