Three bulls were given twice daily a diet of grass silage and barley (70:30) on a DM basis. Manual evacuation of rumen contents was made before feeding, 3, 6 and 9 h after feeding. The interval between two subsequent evacuations was 27 h. Particle size distribution in the digesta was determined by wet-sieving. The greatest diurnal variation in ruminal NDF pool occurred in the 3-mm fraction. Small particles (0.04–0.2 mm) contained less NDF and cellulose and more ADL compared with larger particles. Potential NDF digestibility determined by 288 h nylon bag incubation declined as particle size decreased except for the 0.04-mm fraction. NDF digestibility was positively correlated with the proportion of cellulose in NDF and negatively with that of ADL and hemicellulose. Chemical composition of rumen particulate DM was comparatively unaffected by the time after feeding but NDF digestibility decreased with time when averaged across particle sizes. Particle-associated CM Case and xylanase activities increased gradually as the particle size decreased. The activities were 3 to 9-fold higher in the 0.04-mm fraction than in the other four fractions. There was a close positive relationship between enzyme activities and content of neutral-detergent solubles in particles.