AbstractAcetaldehyde, the main biological metabolite of ethanol, is nowadays considered to mediate some ethanol‐induced effects. Previous studies on alcohol effect attenuation have shown that D‐(–)‐penicillamine (3‐mercapto‐D‐valine), a thiol amino acid, acts as an effective agent for the inactivation of acetaldehyde. In the study reported here, laboratory rats were treated with ethanol and D‐(–)‐penicillamine at different doses looking for the interaction (in vivo) of D‐(–)‐penicillamine with metabolically formed acetaldehyde following a condensation reaction to form the stable adduct (4S,2RS)‐2,5,5‐trimethylthiazolidine‐4‐carboxylic acid (TMTCA). A novel and rapid procedure based on liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) was developed for quantification and reliable identification of TMTCA in different rat tissues, including plasma, liver and brain. Firstly, plasma was obtained from whole blood. Then, proteins were precipitated from plasma, brain and liver extracts with acetonitrile and the clarified extracts diluted 10‐fold. A 20 µL aliquot of the final extracts was then analyzed using an Atlantis C18 5 µm, 100 × 2 mm column which was connected to the electrospray source of a LC/triple quadrupole mass spectrometer. The analyte was detected in positive ion mode acquiring four MS/MS transitions in selected reaction monitoring (SRM) mode.The method has been validated and it has proved to be fast, reliable and sensitive. The accuracy and precision were evaluated by means of recovery experiments from plasma, liver and brain samples fortified at two concentration levels obtaining satisfactory recoveries in all cases: 95 and 105% in plasma (at 10 and 100 ng/mL, respectively), 79 and 89% in brain (100 and 1000 ng/g), 85 and 99% in liver (100 and 1000 ng/g). Precision, expressed as repeatability, was in all tissues analyzed lower than 17% at the two concentrations tested. The estimated detection limits were 1 ng/mL in plasma, 4 ng/g in brain and 5 ng/g in liver. The limit of quantitation objective (the lowest concentration that was validated with acceptable results) was set up at 10 ng/mL for plasma and 100 ng/g for brain and liver tissue.The reliable identification of the analyte was ensured by the acquisition of four transitions and by their ion abundance ratio measurement. Due to its excellent selectivity and sensitivity, the method developed in this work provides an excellent tool for the specific determination of this cyclic amino acid in biological samples. Copyright © 2007 John Wiley & Sons, Ltd.