Abstract: Four medium-frame steer calves (269 kg BW) with cannulas in the rumen, abomasum and proximal duodenum were used in a 4 H 4 Latin square experiment. Steers were allowed ad libitum access to the basal diet, offering 120% of the previous days DMI. Treatments consisted of infusing sodium caseinate (300 g d ¹) into: 1) rumen, via the ruminal cannula; 2) abomasum, via ruminal cannula; 3) abomasum, via the abomasal cannula and 4) proximal duodenum, via the duodenal cannula. Dry matter intake averaged 97 g kg^-1 BW^.75 and was not affected (p>0.20) by treatments. There were no treatment effects (p>0.20) on flow of OM, NDF and ADF to the small intestine. As expected due to the high ruminal degradability of casein (86%), flow of nonammonia N to the small intestine was greater (p<0.01) when casein was infused postruminally then when it was infused ruminally. Casein infusion did not affect (p>0.20) ruminal degradability of dietary N. Observed ruminal degradability of dietary N was in close agreement with expected (49 vs 51%, respectively) based on the National Research Council�s Level 1 model. There were no treatment effects (p>0.20) on ruminal NDF digestion. However, ruminal ADF digestion was greater (5%, p<0.05) when casein was infused postruminally then when it was infused ruminally. As expected, postruminal casein infusion increased postruminal digestion of the OM (21%, p<0.05) and N (12%, p<0.01). There were no treatment effects (p>0.20) on total tract digestion of OM, N, ADF, NDF and GE. Casein infusion did not influence (p>0.20) flow of chyme to the small intestine or ruminal turnover. Flow of chyme to the small intestine was primarily a function of DMI (chyme, L = 5.3 + 12.1DMI; R² = 0.50). Postruminal casein infusion increased (75%, p<0.01) the soluble N content of duodenal chyme, but it did not affect (p>0.20) its tonicity, averaging 264 mOsm. The relationship between tonicity and passage rate of chyme from the abomasum was small ( R² = 0.05 ). Casein infusion did not affect (p>0.20) ruminal DM content or liquid volume, averaging 17.5 g DM kg^-1 BW^.75 and 471 g kg^-1 BW^.75, respectively. There were no treatment effects (p>0.20) on indigestible NDF intake (averaging 26 g kg^-1 BW^.75) or ruminal NDF fill (averaging 41.1 g kg^-1 BW^.75). Casein infusion did not affect (p>0.20) ruminal pH, but ruminal acetate:propionate molar ratio was greater (p<0.10) when casein was infused ruminally. Ruminal acetate:propionate molar ratio was also greater (p<0.05) when casein was infused into the abomasum versus the proximal duodenum. We conclude that independently of N status, increasing protein supply to the small intestine will not facilitate an increase in DM intake. Instead, limits to maximal DM intake in cattle fed forage-based diets appears to be largely a function of ruminal NDF fill capacity and rates of passage and digestion of NDF.