This pilot study is an effort to elucidate the metabolic fate of dietary eicosapentaenoate in vivo and its influence on arachidonate cyclooxygenation at the renal level. The ultimate objective is to shed light on the biochemical mechanisms responsible for the physiologic effects of marine oil on humans. We found prostaglandin E3 (PGE3) in urine of a female volunteer who ingested 10-50 g/day of MaxEPA fish oil concentrate for 4 years. PGE3, a cyclooxygenase metabolite of eicosapentaenoate, could not be detected in 24-h urine pools from the same subject 16 weeks after fish oil supplementation ended. The appearance of PGE3 was concurrent with a reduction of urinary PGE2. Identification of the trienoic prostaglandin was based on comparison of chromatographic behavior of three distinct derivatives of endogenous PGE3 with that of authentic material, and on selected ion monitoring mass spectrometry.