Mean mouth to cecum transit time determined by the hydrogen breath test after oral lactulose in a group of 10 hyperthyroid patients (nine with regular bowel habit and one with diarrhea) was significantly lower than that observed in 10 euthyroid controls [53 min (SD 15) versus 123 min (SD 12.2), p less than 0.01]. After 5 days of propranolol treatment the patients showed a significant reduction of the heart rate but no modification of transit time [51 min (SD 7.3)].