Orocecal transit time was evaluated in 13 cats diagnosed with hyperthyroidism. Transit was determined by measuring the change in breath hydrogen and methane concentrations following oral administration of a nonabsorbable carbohydrate (lactulose). Transit times before and three to four weeks after treatment of the hyperthyroidism with radioactive iodine were compared. There was a significant prolongation of transit time, as determined by a change in hydrogen concentration, following correction of the hyperthyroidism (p = 0.034). Average transit times and standard errors were 27.7 +/- 3.7 minutes before treatment and 56.5 +/- 12.1 minutes after treatment. Methane was not detected in any of the samples. Hyperthyroidism appears to be associated with an accelerated small intestinal transit time in cats.