Alkylphenols, 4-nonylphenol (NP) and 4-tert-octylphenol (OP), in human urine and plasma samples were analyzed using stir bar sorptive extraction (SBSE) in combination with thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS). The method involved correction by stable isotopically labeled surrogate standards, 4-(1-methyl)octylphenol-d5 (m-OP-d5) and deuterium 4-tert-octylphenol (OP-d). A biological sample was extracted for 60 min at room temperature (25 degrees C) using a stir bar coated with a 500 microm thick polydimethylsiloxane (PDMS) layer. Then, the stir bar was analyzed by TD-GC-MS in the selected ion monitoring (SIM) mode without any derivatization step. The average recoveries in human urine and plasma samples spiked with NP and OP at levels of 0.5 and 10 ng ml-1 were between 95.8 and 99.8% with correction using the added surrogate standards. The limits of quantitation were 0.2 ng ml-1 for NP and 0.02 ng ml-1 for OP. We measured the background levels of NP and OP in five human urine and three human plasma samples from healthy volunteers. NP and OP were not detected in all human urine samples (N.D. < 0.2 ng ml-1 for NP, and N.D. < 0.02 ng ml-1 for OP). However, 0.2-0.3 ng ml-1 for NP and 0.1-0.2 ng ml-1 for OP in human plasma samples were observed by this method.