A gas chromatographic method for the determination of the nicotine metabolite trans-3'-hydroxycotinine is described. The method involves conversion of the metabolite to the tert.-butyldimethylsilyl derivative, chromatography on a fused-silica capillary column, and determination using nitrogen-phosphorus detection or electron ionization mass spectrometry with selected ion monitoring. A structural analogue, trans-3-hydroxy-1-methyl-5-(2-pyridyl)pyrrolidin-2-one (trans-3'-hydroxy-ortho-cotinine), was used as an internal standard. Using selected ion monitoring, good precision and accuracy were obtained for determination of trans-3'-hydroxycotinine in urine over the concentration range 10-10,000 ng/ml. There was a good correlation between concentrations determined by selected ion monitoring and by nitrogen-phosphorus detection in urine of smokers, although low concentrations determined using nitrogen-phosphorus detection tended to be somewhat higher, suggesting some interference from urinary constituents. Concentrations and 24-h excretion of trans-3'-hydroxycotinine in the urine of 22 cigarette smokers are reported and compared to concentrations and excretion of nicotine, cotinine, nicotine 1'-N-oxide, nornicotine, and cotinine N-oxide.