BACKGROUND Quantification of plasma free metanephrines is usually accomplished by HPLC with electrochemical detection, but sample preparation is labor-intensive and time-consuming, run times are long, and interfering substances sometimes obscure the relevant peaks. The aim of this study was to develop a sensitive and specific LC-MS/MS method for plasma free metanephrines. METHODS After solid-phase extraction, chromatographic separation of normetanephrine (NMN) and metanephrine (MN) was accomplished by use of a cyano analytical column. NMN, MN, d(3)-NMN, and d(3)-MN positive ions were detected in the multiple-reaction monitoring mode using the specific transitions m/z 166-->134, 180-->148, 169-->137, and 183-->151, respectively, with an atmospheric pressure chemical ionization source. RESULTS Multiple calibration curves exhibited consistent linearity and reproducibility. Interassay imprecision values (CV; n = 20) for NMN at 0.64, 1.9, and 2.7 nmol/L were 6.6%, 7.8%, and 13%, respectively. Interassay CV for MN at 0.60, 1.2, and 2.1 nmol/L (n = 20) were 9.2%, 6.8%, and 9.8%, respectively. The mean recoveries of NMN and MN relative to the internal standard were 100% and 96%, respectively. The assays were linear between 0.20 and 10.0 nmol/L. Deming regression of HPLC and LC-MS/MS results yielded slopes of 0.93 (95% confidence interval, 0.89-0.98) and 0.89 (0.85-0.93) and y-intercepts of -0.16 and 0.03 nmol/L for NMN (n = 132) and MN (n = 92), respectively. CONCLUSIONS This novel LC-MS/MS approach provides a precise, rapid, and specific alternative method to HPLC for the quantification of the low nanomolar concentrations of free metanephrines in plasma.