Ruminal kinetic parameters of different particle size fractions were estimated using four lactating dairy cows in a 4×4 Latin square experiment. Cows received red clover-grass silage alone diet. Ruminal contents and faeces were divided into six particle size fractions by wet sieving. Indigestible (iNDF) and potentially digestible (pdNDF) neutral detergent fibre were separated by a 12d ruminal in situ incubation. Particles >2.5 mm formed the greatest dry matter, iNDF and pdNDF pools in the rumen. Passage rates of iNDF and pdNDF increased with decreasing particle size. Particle breakdown in the rumen was a relatively slow process (0.041 h^-1). Contribution of particle breakdown to turnover of different particle size fractions of rumen iNDF decreased with decreasing particle size while the contribution of passage increased. Accumulation of iNDF in the rumen was greater than generally observed with grass-based diets and pdNDF was rapidly digested.