The disappearance of Maillard reaction products (MRPs) during ensilage and rumen fermentation was studied. MRPs were prepared by heating mixtures of D-glucose and glycine (GG) or D-xylose and glycine (XG); lyophilised powder (MW > 1000) was subjected to anaerobic incubation for 24h with lucerne juice or buffered rumen fluid. Changes in MRPs were assessed by comparing gel filtration profiles before and after incubation, and the proportion of disappeared MRPs was calculated based on the area under the curve. A portion of MRPs, particularly high-MW fractions, disappeared when incubated with lucerne juice, and the extent was greater (P < 0.01) in XG than in GG. The disappearance of GG was increased (P < 0.01) when ensilage was fortified with added glucose, while that of XG was unaffected with or without the fermentable substrate. Rumen fermentation had little impact on GG and XG, whereas a portion of MRPs disappeared when microbial activity was enhanced by adding lucerne hay to the media. Volatile fatty acid production was unaffected by MRPs in either ensilage or rumen fermentation. These results suggest that the effect on MRPs of anaerobic fermentation may be different between ensilage and rumen digestion. Significant amounts of MRPs would enter the intestine when ruminants are fed hay-based diets rather than silage-based diets. (C) 2000 Society of Chemical Industry.