17β-Nortestosterone (17βN) is illegally used in livestock as a growth promoter and its endogenous production has been described in some animals, such as adult boars. In this paper, the metabolism of 17βN in boars has been studied by gas chromatography/mass spectrometry (GC/MS) in order to identify markers of the exogenous administration. Administration studies of intramuscular 17βN laurate to male pigs were performed. Free, sulphate and glucuronide fractions of the urine samples were separated and the steroids present were quantified by GC/MS. 17βN was detected in some pre-administration samples. After administration, 17βN, norandrosterone, noretiocholanolone (NorE), norepiandrosterone, 5β-estrane-3α, 17β-diol and 5α-estrane-3β,17β-diol were detected in different fractions, being the most important metabolites, 17βN excreted as a sulphate and free NorE. Samples collected in routine controls were also analyzed by GC/MS to identify endogenous compounds. 17βN, norandrostenedione and estrone were detected in almost all the samples. No other 17βN metabolites were detected. According to these results, the detection by GC/MS of some of the 17βN metabolites described above, different from 17βN, could be indicative of the exogenous administration of 17βN to boars. Copyright © 2008 John Wiley & Sons, Ltd.