Considerable evidence has accumulated that feeding riboflavin in excess of nutritional requirements has very little toxicity either for experimental animals or for humans (Rivlin, 1978). There are no reports of riboflavin toxicity studies in ruminants. Most data from rats suggest that dietary levels between 10 and 20 times the requirement (possibly 100 times) can be tolerated safely (NRC, 1987). When massive amounts of riboflavin are administered orally, only a small fraction of the dose is absorbed; the remainder is excreted in the feces. The lack of toxicity is probably due to saturation of the intestinal transport system (Christensen, 1973). Also, capacity of the tissues to store riboflavin and its coenzyme derivatives appears to be limited.