Inclusion of CM in dairy rations decreases plasma urea concentration (Martineau et al., J. Dairy Sci. 97:1603).The objective of this study was to compare the effects of feeding diets containing SBM, CM, HPDDG or WDDGS as single protein supplement on urea kinetics. Eight rumen-fistulated Holstein cows were used in a replicated 4 × 4 Latin square with 14-d periods. Dietary treatments consisted of inclusion of these protein in diet: SBM (13.7%), CM (20.8%), HPDDG (20.4%) or WDDGS (22.8%). The 4 diets were isonitrogenous (17.2%CP) and isoenergetic (1.56 Mcal NE_L/kg DM; NRC 2001), with a fixed forage:concentrate ratio of 62:38. From d 10–13, total collection of urine was performed. On d 12, cows were infused in a jugular vein ¹⁵N₂-urea (0.5 mmol/h) and urine spot samples were collected after 24 h of infusion; enrichment of ¹⁵N₂-urea was determined to estimate urea kinetics. Differences among treatments were analyzed using adjusted Tukey-Kramer multiple comparisons test. Results are given as LSM ± SEM for SBM, CM, HPDDG and WDDGS, respectively. The N intake and milk N were similar between treatments (659, 660,657, and 665 ± 23 g/d and 1.13, 1.15, 1.15, and 1.13 ± 0.01 kg/d; Maxin et al., JDS 96:7806). The RDP supply (NRC 2001) averaged 2622, 2672, 2110 and 2959 g/d. Urea production (UER) tended (P = 0.08) to be lower for WDDGS vs. SBM (353 vs. 409 ± 18 g urea-N/d) whereas CM (391) and HPDDG (405) were intermediate. Urea gut entry rate (GER) averaged 265, 272, 267 and 210 ± 18 g urea-N/d, with only WDDGS being lower (P < 0.05) than the other diets. The proportion of GER/UER was also lower (P < 0.05) for WDDGS: 0.65, 0.69, 0.65, and 0.59 ± 0.02. Urinary N excretion was lower (P < 0.06) for CM (118 ± 6 g urea-N/d) than HPDDG (139), WDDGS (144) and SBM (144). Cows fed WDDGS had a lower urea production and GER, but urinary excretion was similar to HPDDG and SBM, in disagreement with estimated higher RDP supply. The intermediate urea production for the CM supplement coupled with the numerically higher proportion returned to the gut lead to a lower urinary urea-N excretion which can explain the decreased plasma urea concentration also observed in this study: 8.5, 7.8, 8.8, and 8.9 ± 0.4 mM (Maxin et al., J. Dairy Sci. 96:7806).