The aim of this study was to utilize the lactulose/mannitol test for assessment of intestinal permeability (IP) in preruminant calves. IP was increased experimentally by administration of Indomethacin. Both sugar markers were determined simultaneously as silyl-derivates in 5 h urinary production using the gas liquid chromatography technique. The index of IP was determined as the ratio between urinary recovery (%) of 10 g lactulose to 5 g D-mannitol. The IP index in control calves was 0.242±0.048. Doses of 20 and 60 mg Indomethacin did not significantly affect IP, but the IP index was significantly enhanced in calves receiving 120 mg of drug (0.541±0.091; P<0.01). Results of this study show that ratio of lactulose to D-mannitol in urine reflected the treatment by the highest dose of Indomethacin and that the test may be used for determination of IP in preruminant calves.