Abstract: A mixture of D-neopterin, biopterin, pterin, 7,8-dihydropterin, 6R-L-erythro-5,6,7,8-tetrahydrobiopterin and 6S-L-erythro-5,6,7,8-tetrahydrobiopterin was analyzed by HPLC at 40°C on a column (25 cm x 4.6 mm) of Cosmosil 5C18 equipped with a guard column (5 cm x 4.6 mm) of the same packing. Elution (1 ml/min) was effected with 0.1 M phosphate buffer of pH 3 containing 5% methanol, 3 mM sodium octylsulfate, 0.1 mM Na2EDTA and 0.1 mM ascorbic acid. Post-column derivatization was achieved by mixing the eluate with a stream of 5 mM NaNO2 (1 ml/min) before passing through a reaction coil (no dimensions given) at 80°C; fluorescent detection was performed at 440 nm (excitation at 350 nm). The calibration graphs were linear from 0.02-5 ng of each compound injected, with detection limits of 10^-20 pg. Recoveries from tissues ranged from 84.7-99.6% and the within-assay RSD were 0.7-3.7%. The method was applied to the analysis of homogenates of rat liver, kidney and 6 regions of the brain.