. 2009. Effect of dry matter intake on visceral organ mass, cellularity, and the protein expression of ATP synthase, Na /K -ATPase, proliferating cell nuclear antigen and ubiquitin in feedlot steers. Can. J. Anim. Sci. 89: 253Á262. Twenty-four steers [46797.2 kg initial body weight (BW)] predominately of Angus breeding were used to determine the effect of dry matter intake (1.25, 1.50, 1.75, and 2.00% of BW) on visceral mass, cellularity, and the protein expression of ATP synthase, Na ' /K ' -ATPase, proliferating cell nuclear antigen (PCNA) and ubiquitin. There were linear increases (P50.05) in weights of total viscera, total digestive tract, liver, kidney, heart, lung, spleen, rumen, and abomasum with increasing dry matter intake (DMI). Protein concentration decreased linearly (PB0.05) in small intestinal mucosa as DMI increased. PCNA expression increased linearly (PB0.01) in liver as DMI increased. PCNA expression was affected quadratically (PB0.05) in pancreas and small intestinal mucosa with an increase when DMI increased from 1.25 to 1.75% of BW, and a decrease when DMI increased from 1.75 to 2% of BW. ATP synthase, Na ' /K ' -ATPase, and ubiquitin expression in pancreas and ubiquitin expression in small intestinal mucosa increased linearly (PB0.05) as DMI increased. These results indicate that increasing DMI increases the mass of visceral organs and carcass and influences expression of proteins influencing energy utilization and efficiency in pancreas, small intestine, liver, and sternomandibularis muscle. . Incidence de l'ingestion de matie`re se`che sur la masse des visce`res, sur la cellularite´ainsi que sur l'expression prote´ique de l'ATP synthase, de la Na /K -ATPase, de l'antige`ne nucle´aire de prolife´ration cellulaire et de l'ubiquitine chez les bouvillons de boucherie. Can. J. Anim. Sci. 89: 253Á262. Vingt-quatre bouvillons de boucherie (46797,2 kg de poids corporel initial) principalement d'ascendance Angus ont servi a`pre´ciser l'incidence de l'ingestion de matie`re se`che (1,25, 1,50, 1,75 et 2,00 % du poids corporel) sur la masse des visce`res, sur la cellularite´ainsi que sur l'expression prote´ique de l'ATP synthase, de la Na ' /K ' -ATPase, de l'antige`ne nucle´aire de prolife´ration cellulaire (PCNA) et de l'ubiquitine. Le poids total des visce`res, le poids total du tube digestif ainsi que le poids du foie, des reins, du caeur, des poumons, de la rate, du rumen et de la caillette augmentent line´airement (P 50,05) avec l'ingestion de matie`re se`che. La concentration de prote´ines diminue de fac¸on line´aire (P B 0,05) dans la muqueuse de l'intestin greˆle quand l'ingestion de matie`re se`che augmente. L'expression du PCNA dans le foie enregistre une hausse line´aire (PB0,01), quand l'ingestion de matie`re se`che augmente. L'expression du PCNA dans le pancre´as et la muqueuse de l'intestin greˆle subit une influence quadratique (PB0,05) lorsque l'ingestion de matie`re se`che passe de 1,25 % a`1,75 % du poids corporelle, puis diminue quand elle passe de 1,75 % a`2 % du poids corporel. L...