Abstract: Platelet-activating factor (PAF) was for the first time confirmed to exist in a lower animal, earthworm (Eisenia foetida). It amounts to 10. 7±6. 1 pmol/g wet body weight, andvaried seasonally. Phospholipid analysis revealed that 1-O-alkyl-2-acyl-sn-glycerophosphocholine, astored form of PAF precursor, accounted for 61.4% of the choline glycerophospholipids. Two kindsof enzyme activities operating in PAF generation in mammalian cells were also detected from this speci-es. The PAF level increased markedly under some injurious stimuli such as cutting,and pricking. Theresults suggest that PAF may be a primary mediator involved in pathological and physiological reac-tions even in lower animal like earthworm. The findings also cast a new light on the mechanisms under-lying the antihypertensive and other effects of the Chinese medicinal earthworm, pheretima.