A sensitive and selective method for the analysis of ibuprofen enantiomers by LC-MS/MS was developed and validated for the purpose of application in pharmacokinetic studies in small experimental animals. Aliquots of 200 L plasma were submitted to liquid-liquid extraction with hexane/diisopropylether (50:50 v/v) in acid pH. Separation was accomplished in a Chirex (R) 3005 (250 x 4.6 mm, 5 m) column at 25 degrees C with a mobile phase that consisted of 0.01 M ammonium acetate in methanol at a flow rate of 1.1 mL/min. The mass spectrometer consisted of an ESI interface operating at negative ionization mode and multiple reaction monitoring. The transitions 205 > 161 and 240 > 197 were monitored for ibuprofen enantiomers and fenoprofen (internal standard), respectively. Method validation included the evaluation of the matrix effect, stability, linearity, lower LOQ, within-run and between-run precision, and accuracy. The lower LOQ was 25 ng/mL for each ibuprofen enantiomer, and the calibration curves showed good linearity in the range 0.025-50 g/mL. The method was successfully applied in the investigation of pharmacokinetic disposition of ibuprofen enantiomers in rats treated orally with 25 mg/kg of the racemate. Enantioselective kinetic disposition was observed with accumulation of (+)-(S)-ibuprofen in rats following single oral administration.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Univ Sao Paulo, Fac Ciencias Farmaceut Ribeirao Preto, Dept Anal Clin Toxicol & Bromatol, BR-05508 Sao Paulo, BrazilUniv Estadual Paulista, Fac Ciencias Farmaceut Araraquara, Dept Principios Ativos Nat & Toxicol, BR-14801902 Araraquara, SP, BrazilUniv Estadual Paulista, Fac Ciencias Farmaceut Araraquara, Dept Principios Ativos Nat & Toxicol, BR-14801902 Araraquara, SP, BrazilFAPESP: 09/17532-0