Gastric emptying, mouth-to-cecum transit, and whole gut transit of a solid-liquid meal was measured in 20 hyperthyroid patients and in 30 control subjects by using scintigraphic techniques, the hydrogen breath test, and stool markers. In the hyperthyroid patients predefined gastrointestinal symptoms were determined and related to gastrointestinal transit. There was no significant overall difference of gastric emptying between the hyperthyroid patients and the control subjects, whereas both mouth-to-cecum transit time (mainly reflecting small intestinal transit) and whole gut transit time (mainly reflecting large intestinal transit) were significantly accelerated in hyperthyroid patients as compared to the control subjects (P less than 0.005). Mouth-to-cecum transit in hyperthyroid patients with diarrhea tended to be more rapid than in those without diarrhea (P = 0.094) and the T3 concentrations were inversely correlated with mouth-to-cecum transit time. It is concluded that in thyrotoxicosis 1) small and large intestinal transit is accelerated, while gastric emptying remains unchanged and 2) rapid intestinal transit is likely to be one factor among others implicated in the generation of diarrhea.