Twenty-eight lambs averaging 45 kg live weight were divided among four roughage dietary treatment : grass hay, corn silage, alfalfa and oat hay. The diets contained a minimum of 12% crude protein and provided 10% over the required energy for maintenance. Lambs were fed twice a day (0800 and 1700) for 28 days. Animals were weighed at 10-d intervals at 0700 prior to being fed that morning. At the end of the feeding period the lambs were slaughtered and the weights of fat-free gastrointestinal tract segments, their content and other organs were taken and expressed as gfkg of empty body weight (EBW) Sampled of the rumen and small intestine were separated into mucosa] and nonmucosal (serosal) fractions. No single forage diet consistently increased the weights of all visceral organs. However, lambs fed the 70% alfalfa hay has 12% heavier liver and lung weights (p < .05) compared to lambs receiving other treatments and 11% heavier small intestine weights compared to the oat and silage diets. The grass hay diets resulted in lambs with 55% heavier cecum dry weights (P < . 05) compared to other roughage sources . Lambs fed silage diets had 7% heavier rumen mucosa (wet and dry) weights . The roughage sourse had no effect on total rumen, omasum, large intestine and total tract weights or contents.