The low levels of S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy) in plasma can be measured by formation of the fluorescent isoindole derivatives of these compounds. The procedure involves an initial separation of AdoMet and AdoHcy in deproteinized plasma by HPLC on a C-8 column followed by derivatization with naphthalenedialdehyde and cyanide for 10 min at pH 9.0. The fluorescent derivatives of AdoMet and AdoHcy are then chromatographed by HPLC on a C-18 column monitored with a fluorescence monitor. The formation of the isoindole goes to 85% completion and the overall recovery of standards added to plasma is about 65%. Correction for recovery is made by addition of known quantities of AdoMet and AdoHcy to plasma. Normal values (+/- SE) for AdoMet were 102.7 nM +/- 9.9 and for AdoHcy were 22.7 +/- 3.1.