The EI mass spectra of four tert-butyldimethylsilyl ether derivatives of the major metabolite of prostaglandins F1 alpha and F2 alpha (PGF-M) are presented and discussed. Proposed ion assignments and fragmentation pathways are based on substituent shifts, on data from a deuterium-labeled methoxime analog, and on the analysis of collision-induced dissociation spectra of selected ions. Fragment ions suitable for identification and quantification work are proposed.