Sephadex G-100 gel filtration of urine from male Wistar rats revealed 3 protein peaks: peak I (eluted with the void volume of the column), peak II (Mr between 67 000 and 43 000), and peak III (Mr about 13 700). Nephrotoxic compounds were given as a single i.p. injection. Peak I, and especially peak II were significantly increased in 24-h urine samples from rats receiving mercuric acetate (1.58 mg/kg), mercuric trifluoroacetate (MTFA) (2.22 mg/kg), sodium ethylmercurithiosalicylate (EMTSA) (20.2 mg/kg), sodium tetrathionate (250 mg/kg), ammonium fluoride (18.5 mg/kg), paramomycin sulfate (800 mg/kg), ochratoxin A (5 mg/kg) and cis-platinum (4 mg/kg). It is concluded that gel filtration of urine can be used as a method to detect nephrotoxicity in rats.