A low quality Eragrostis curvula hay was supplemented with 30 or 80 g casein in the rumen with or without 70 g casein in the abomasum, and a medium quality E. curvula hay with 30 g casein in the rumen with or without 70 g casein in the abomasum. Six rumen and abomasal cannulated sheep were used in a Latin-square design. Hay intake and ADG were improved by an increase in hay quality and casein administration to the abomasum but not by an increase in casein supplementation to the rumen. Similarly, non-ammonia N flow to the abomasum was increased by an increase in hay quality and postruminal addition of casein but not by an increase in casein administered to the rumen. It was suggested that either the release of C-skeletons from a natural protein source in the rumen does not contribute much to microbial protein formation or that the protein source should not be highly soluble, i.e. slow releasing, to be effective. In contrast, microbial protein production should be increased by rumenprotected protein via recirculation of urea whether forage quality is low or high, and thereby cause a production response