For the first time, a rapid, sensitive and simple liquid chromatography/tandem mass spectrometry (LC-MS/MS) method using an atmospheric pressure chemical ionization (APCI) source for the quantification of PD168393 in rat serum was developed and validated. Serum samples were pretreated with methanol for protein precipitation. The chromatographic separation was performed on a Jupiter-C5 column (250 mm × 2.0 mm i.d.) pre-equilibrated with 0.1% formic acid. The tandem mass spectrometer was tuned in the multiple reaction monitoring mode to monitor the m/z transitions 369/313 for PD168393 and m/z 343/308 for the internal standard triazolam, using positive ion mode. The MS/MS response was linear over the concentration range from 2 ng/mL to 5000 ng/mL, with a lower limit of quantification (LLQ) of 2 ng/mL. At the lowest quality control (4 ng/mL), the intra- and inter-day precisions (CV%) for PD168393 were less than 10% and the accuracies were between 92% and 111%. The validated method can be used in most or all stages of the screening and optimizing process for future method validation of pharmacokinetic studies. © 2008 Elsevier B.V. All rights reserved.