In sacco degradation curves of NDF from small (SP) and large particles (LP) of rumen contents, collected in dairy cows before or after grazing, were compared. LP and SP were defined as ruminal particles retained, or passing a 1 mm-pore-sieve, respectively. Degradation curves were fitted with a first order kinetics model. SP, compared to LP, contained a lower proportion of digestible NDF. Fractional degradation rates of NDF were lower for SP than for LP after the meal, and were not different between SP and LP before the meal. These results questions about representations of SP and LP in the rumen models.