In vitro studies have shown that estrogen and progesterone can affect the contractile response and myoelectric activity of the gastrointestinal smooth muscle. The present study was designed to investigate the effect of sex steroid hormones on gastric emptying and gastrointestinal transit were assessed in rats 15 min after intragastric instillation of a test meal containing charcoal and 51Cr. Gastric emptying was determined by measuring the amount of labeled chromium contained in the small intestine as a percentage of the initial amount received. Gastrointestinal transit was evaluated by calculating both the geometric center of distribution of the radiolabeled marker and the charcoal transit in the intestine. The experimental animals included diestrus rats; ovariectomized rats treated with vehicle, estradiol, and/or progesterone; and normal male and orchiectomized rats treated with vehicle or testosterone. Female rats in diestrus had a slower gastric emptying and a lesser geometric center value than ovariectomized rats. Estradiol inhibited gastric emptying but did not affect gastrointestinal transit. Progesterone increased gastric emptying. Progesterone at lower dose (10 mg/kg) decreased the geometric center compared with higher doses (20 or 40 mg/kg) or vehicle controls. A mixture of estradiol (10 micrograms/kg) and progesterone (20 mg/kg) inhibited gastric emptying to a similar degree as estradiol (10 micrograms/kg) did. Testosterone had no influence on gastric emptying or gastrointestinal transit. These results suggest that estradiol and a mixture of estradiol and progesterone inhibit, whereas progesterone enhances, gastric emptying. Testosterone did not play a role in gastrointestinal motility.