Urinary metabolites in rats treated with p-chloronitrobenzene were identified by gas chromatography-mass spectrometry. A single dose of 100 mg/kg body wt p-chloronitrobenzene was administered intraperitoneally to male Sprague-Dawley rats and urine samples were collected from the 8th to 24th hour after the administration. Urinary metabolites were extracted with diethylether at pH 1.0 and pH 10.0 from urine samples hydrolyzed with acid and base and from intact urine samples. Aliquots of the ethereal extracts were injected into a gas chromatograph-mass spectrometer. Nine substances were identified: p-chloroaniline, 2,4-dichloroaniline, p-nitrothiophenol, 2-chloro-5-nitrophenol, 2-amino-5-chlorophenol, p-chloroformanilide, 4-chloro-2-hydroxyacetanilide, a small amount of p-chloroacetanilide and traces of unchanged p-chloronitrobenzene.