Six Holstein cows fitted with ruminal cannulae were used in two simultaneous 3 x 3 Latin squares to study the effects of protein supplements on ruminal fermentation and in situ crude protein degradability. Cows rotationally grazed a winter oats (Avena sativa L.) pasture and were supplemented with one of three concentrate supplements: 1) low protein sunflower meal (L-SM); 2) high protein sunflower meal (H-SM); or 3) high protein feather meal (H-FM). Concentrates (6.5 kg/d) were offered in equal portions twice daily during milking. Ruminal pH and total volatile fatty acids concentration were unaffected by treatments. Supplementation with L-SM and H-FM decreased ruminal NH3-N concentration compared with H-SM. The concentrate with feather meal had lower effective rumen degradability of crude protein than concentrates containing sunflower meal. Effective rumen degradability of crude protein of pasture averaged 82.7%. Thirty-six multiparous Holstein cows (71 d in milk) were used in a complementary experiment to study the effect of treatments on intake, milk yield, and milk composition. Pasture (13.2 kg/d) and total (19.6 kg/d) dry matter intake (estimated using Cr2O3 as fecal marker) and milk yield (20.5 kg/d) were unaffected by level or source of protein supplemented. Intake of rumen undegradable protein in grazing dairy cows was higher when the amount of sunflower meal was increased or when feather meal was used in the supplement. However, higher rumen undegradable protein intake did not increase milk production, suggesting that rumen undegradable protein was not limiting for cows on pasture producing less than 22 kg of milk.