Finishing barrows (average initial weight 55.5 ±2.4 kg) were used to determine the effects of high dietary fiber or protein on performance, visceral organ weights and large intestine microbial populations and to monitor the duration of regression of swine visceral organ mass and microbial populations to control values following trans fer from the high fiber or high protein diet to the control diet Four pigs from each diet were killed on d 17, 34, 48 and 66. From d 34 until slaughter 14 and 32 d later, all remaining pigs were fed the control diet ad libitum. High fiber resulted In significantly higher relative weight of the total gastrointestinal tract after 34 d and higher relative stomach weight up to d 48. Compared with the control diet, the high protein diet resulted in increased relative liver and kidney weights up to d 48. The number of proteolytic and cellulolytic bacteria increased in the colon contents of pigs fed a high protein or high fiber diet, respectively, but declined to below control values within 14 d of transferring pigs from the high protein or high fiber diet to the control diet. The results Indicate that diet composition plays a more specific role in visceral organ hypertrophy than can be ex plained by the normal relative changes in organ size as body weight increases. Thus, high dietary fiber and protein may indirectly increase the animal's maintenance require ment by causing a repartitioning of nutrients from the edible carcass to the visceral organs.