tert.-Butyldimethylsilyl ethers of eighteen prostaglandins (PGs) and thromboxane have been analyzed by combined packed-column or fused-silica capillary gas chromatography-mass spectrometry (GC-MS). Except for thromboxane B2 and PGs of the 3 series the [M - 57] ion, the loss from the molecular ion of the tert.-butyl radical, is generally the major peak in the spectra of PGs. This high abundance of large fragments provides suitable criteria for the employment of these derivatives in GC-MS selected ion monitoring for rapid quantity estimation of PGs from biological samples. PGs from rat stomach fundus were rapidly quantitated in a single chromatogram using this procedure.