A 4 × 4 Latin square was conducted with four rumen cannulated cows to investigate responses to isonitrogenous supplementation of grass silage-cereal diet with rapeseed meal (RSM) or peas supplied either dried or ensiled. Omasal canal flow measurements showed that protein supplements increased non-ammonia N flow to the intestine, though pea supplements less than RSM. Plasma concentrations of branched chain and essential amino acids were also increased by both RSM and pea supplements. All protein supplements increased yields of milk and milk constituents compared to control, but these increases seemed to be less with peas than with RSM.