Changes in the fatty acid composition of phospholipid and triglyceride fractions in Spirometra erinaceieuropaei plerocercoids were investigated after 0, 0.5, 1, 3, and 6 hr incubation at 10 C and 37 C with physiological saline containing 5 mM arachidonic acid and 10 mg/ml bovine serum albumin, pH 7.0. At 37 C, arachidonic acid was absorbed and incorporated rapidly into the triglyceride fraction (over 14.4% in composition), and decreased after 2-3 hr; at 10 C, the amount of triglyceride increased slowly and continued to a maximum of 12.9% during 6 hr of incubation. We used a simplified method to extract and purify prostaglandins from the plerocercoid of S. erinaceieuropaei. Prostaglandins were quantified using gas chromatography-mass spectrometry. Prostaglandin E2, PGD2, PGF2alpha, and 6-keto-PGF1alpha were detected under different incubation conditions. In the dose-dependent experiment, PGD2 was detected in plerocercoids incubated with 0.5, 1, 2, and 5 mM arachidonic acid, pH 7.0, at 25 C; PGE2 was detected with 2 and 5 mM arachidonic acid. In the time-dependent experiment, where plerocercoids were incubated with 5 mM arachidonic acid, pH 7.0 at 25 C, PGF2alpha was first detected at 15 min; thereafter, 6-keto-PGF1alpha was detected at 30 min and PGD2 and PGE2 were detected at 1 hr. Thromboxane B2 was not detected in either the dose-dependent or time-dependent experiments, and only PGE2 was detected in the incubation medium with 5 mM arachidonic acid at 1 hr. These results reveal that when plerocercoids change from reptilian to mammalian hosts, they are able to absorb and modify arachidonic acid bound to albumin and generate prostaglandins under suitable conditions. Prostaglandins exhibit potent biological functions for immunoresponses that may be relevant to parasitism and the success of larva migrans in S. erinaceieuropaei.