Breath hydrogen excretion over a period of three hours was measured to evaluate carbohydrate malassimilation in healthy cats treated orally with antibiotics. Both an absorbable carbohydrate (xylose) and a non-absorbable carbohydrate (lactulose) were administered during the tests to evaluate the changes in the intestinal mucosa and the population of bacteria within the intestinal lumen. Overall, the effects of oxytetracycline and metronidazole on breath hydrogen excretion were not significantly different. However, the treatment effect with an antibiotic did significantly change breath hydrogen excretion after xylose administration (P<0.05) within groups. Similarly, with each antibiotic, breath hydrogen excretion was affected significantly (P<0.001) by the time after the administration of the carbohydrate. Treatment with each antibiotic also interacted significantly with this time effect (P<0.05) within groups. After lactulose administration, there was a trend within groups for the type of antibiotic to interact with the treatment effect on breath hydrogen excretion (P = 0.09). After oxytetracycline treatment, more hydrogen was exhaled during the first 120 minutes after lactulose administration than in the pre-antibiotic test, whereas after metronidazole treatment, less hydrogen was exhaled between 60 and 180 minutes after lactulose administration. After treatment with either oxytetracycline or metronidazole, more hydrogen was exhaled after xylose administration. Obligate anaerobes could be isolated from samples of small intestinal fluid obtained endoscopically after oxytetracycline treatment, but they could not be isolated after treatment with metronidazole.