A sensitive method for the quantification of lidocaine and its metabolites, monoethylglycinexylidide (MEGX) and glycinexylidide (GX), in animal plasma using high-performance liquid chromatography combined with electrospray ionization mass spectrometry is described. The sample preparation includes a liquid-liquid extraction with methyl tert-butylmethyl ether after addition of 2 M sodium hydroxide. Ethyl methylglycinexylidide (EMGX) is used as an internal standard. For chromatographic separation, an ODS Hypersil column was used. Isocratic elution was achieved with 0.0 1 M ammonium acetate and acetonitrile as mobile phases. Good linearity was observed in the range of 2.5-1000 ng ml(-1) for lidocame in both dog and horse plasma. For MEGX, linear calibration curves were obtained in the range of 5-1000 ng ml(-1) and 20-1000 ng ml(-1) for dog and horse plasma, respectively. In dog and horse plasma good linearity was observed in the range of 200-1500 ng ml(-1) for GX. The limit of quantification (LOQ) in dog plasma for lidocaine, MEGX and GX was set at 2.5 ng ml(-1), 20 ng ml(-1) and 200 ng ml(-1), respectively. For horse plasma a limit of quantification of 2.5 ng ml(-1), 5 ng ml(-1) and 200 ng ml(-1) was achieved for lidocaine, MEGX and GX, respectively. In dog plasma, the limit of detection (LOD) was found to be 0.8 ng ml(-1), 2.3 ng ml(-1) and 55 ng ml(-1) for lidocaine, MEGX and GX, respectively. In horse plasma the LOD's found for lidocame, MEGX and GX, were 1.1 ng ml(-1), 0.5 ng ml(-1) and 13 ng ml(-1), respectively. The method was shown to be of use in pharmacokinetic studies after application of a transdermal patch in dogs and after an intravenous infusion in horses. (c) 2007 Elsevier B.V. All rights reserved. ; Peer reviewed