Three ruminally cannulated dry cows were used in a Latin square design to investigate the relationship between microbial fibrolytic enzyme activities and in succo cell wall degradation of two gramineous hays, in which cell wall content ranged from 510 g kg-' DM for the regrowth to 687 g kg-' DM for the late harvested hay. Animals were fed twice daily a diet consisting of wheat straw, cocksfoot hay and ground barley in the ratios 10 : 90 : 0 (w/w), 10 : 60 : 30 (w/w) and 10 : 30 : 60 (w/w). For each diet and each hay, the in succo degradation of cell wall polysaccharides and phenolic acids was determined. After 2 h and 23 h incubation time in the rumen, pH was measured in the bags, and fibrolytic activities (xylanase, avicelase, fl-glycosidases) of the microbial population colonising hays in succo were measured. Supplementation above 30% barley decreased the degradation rate of the cell wall polysaccharides, concomitantly with a decrease in polysaccharidase activities. The decrease in degradation rate was more marked for the regrowth than for the late harvested hay, for hemicelluloses than for cellulose and for ferulic than for p-coumaric acid. These differences did not appear to be related to microbial activities, which were similar between hays and between polysaccharidases, but rather to differences in accessibility of cell wall components to microbial enzymes, related to the composition of the forage and the cell wall architecture.