The intestinal absorption and biotransformation of ethinylestradiol and of ethinylestradiol glucuronide in rats were studied using a jejunal loop preparation in situ. Radioactivity associated with ethinylestradiol almost completely appeared in the venous outflow within 80 min. Forty two percent was present as unchanged compound, 56% in the glucuronide fraction and 2% as sulfate ester. Absorption of ethinylestradiol glucuronide was about 20 times slower than absorption of the nonconjugated molecule. The data indicate, that intestinal absorption of ethinylestradiol involves several superimposed kinetics and that glucuronidation in the gut may result in a decreased bioavailability of the compound.