Gas chromatography/electron capture negative ion mass spectrometry of eicosanoids, as pentafluorobenzyl (PFB) ester, methyl oxime (where applicable), trimethylsilyl (TMS) ether derivatives, is reported using a double-focusing instrument of trisector configuration. Sub-picogram detection limits were observed for prostaglandins E1, E2 and F2 alpha during selected ion monitoring (SIM) of [M-PFB]- ions. Selected reaction monitoring (SRM) of [M-PFB]-----[M-PFB-TMSOH]-, occurring in the first field-free region, was of modest sensitivity, reflecting the stability of the [M-PFB]- ions. The leukotriene B4 (LTB4) derivative was successfully analyzed by SIM at the low-picogram level. In this instance, the fragmentation [M-PFB]-----[M-PFB-TMSOH] occurred in high yield in the first field-free region. The advantageous improvement in selectivity of detection that may be achieved with SRM was evident during the analysis of a serum extract for LTB4.