The separation characteristics of the trimethylsilyl ether derivatives of various naturally occurring and synthetic pteridines on a apolar glass capillary column, together with their mass spectra, permit their identification and quantitation in biological samples. Examples are given of the determination of the ratio of monapterin to neopterin in urine, of monitoring excreted pterin metabolites after a loading test with 6-methyltetrahydropterin in urine and of structure elucidation of lumazines , previously unknown in man. 6- Methylisoxanthopterin was shown to be the main metabolite in urine after administration of 6-methyl-5,6,7,8-tetrahydropterin. Biolumazine and 2'- deoxysepialumazine were found in human faeces after administration of ( 6R ,S)-5,6,7,8-tetrahydro-L-erythro-biopterin.