This study aimed to assess separately the effects of gender, age, and body mass index on gastric emptying, small intestinal transit, and colonic transit times of a meal containing 99mTc-labeled cellulose fiber and 2- to 3-mm 111In-labeled plastic particles. Seventeen healthy young subjects (nine men; eight women; age 21-27 years; body mass index 18.4-25.1 kg/m2) and 16 healthy older subjects (eight men; eight women; age 55-74 years; body mass index 19.8-36.0 kg/m2) were studied. All transit variables were unaffected by gender. The older subjects had a slower mean colonic transit time of radiolabeled plastic particles than the young subjects (P < 0.05). Age did not affect mean gastric emptying or mean small intestinal transit times of the radiolabeled markers. An inverse association was found between body mass index and mean gastric emptying time of radiolabeled cellulose fiber (P < 0.02). Body mass index had no influence on other transit variables. The study revealed a considerable intersubject and a somewhat smaller intrasubject variability in mean gastric emptying, mean small intestinal, and mean colonic transit times.