A novel analytical method has been developed for the determination of 4-tert.-octylphenol (OP) and 4-nonylphenol (NP) in laboratory animal feed samples, which involves stir bar sorptive extraction (SBSE) followed by liquid desorption (LD) and column-switching liquid chromatography-mass spectrometry (CS-LC-MS) with solid-phase extraction (SPE). The method required correction by stable isotopically labeled surrogate standards, deuterium 4-tert.-octylphenol (OP-d) and [2H5] 4-(1-methyl)octylphenol (m-OP-d5). A feed sample was homogenized with methanol by ultrasonication. After centrifugation, the supernatant was subjected to extraction for 120 min at room temperature (25 degrees C) using a stir bar coated with polydimethylsiloxane. After the extraction, the analyte was desorbed from the stir bar by LD using acetonitrile. Then, the liquid sample was analyzed by CS-LC-MS with SPE. The average recoveries from laboratory feed samples spiked with OP and NP at 20 ng g(-1) were 99.5 and 103.8%, respectively, with correction using the added surrogate standards. The limits of quantification were 1 ng g(-1) for OP and 5 ng g(-1) for NP in feed sample. The measurement of OP and NP in commercial laboratory animal feed samples resulted in the detection of sub ng g(-1) NP