Three non-lactating cows of about 470 kg BW were fed rations of meadow hay and concentrate (79:21) containing fish meal (ration F) or rapeseed oilmeal (ration R). Total rumen content was evacuated manually before feeding (0 h) and 4 and 8 h after feeding. The interval between two successive evacuations was 7 days. Rate of passage of solid particles from the rumen was measured using Cr mordanted hay, liquid fraction outflow using Co-EDTA as indicators. Particle size distribution in the digesta was determined by wet-sieving using screen mesh sizes 5.0; 1.0 and 0.25 mm. The total amounts of rumen digesta, dry matter and crude protein measured at different times after feeding did not depend on the source of protein in the ration. The potential digestibility of crude fibre, ADF, NDF and ADL was higher (P<0.05) on the ration with fish meal, particularly 8 and 12 h after feeding. The proportion of the different sizes of particles in DM of total digesta was uniform over time after feeding. The proportion of particles smaller than 1 mm was only a little higher in the rumen of cows fed ration F (P<0.05). Crude protein content was higher (P<0.05) in particles larger than 5 mm and of lignin in particles smaller than 1 mm when feeding the fish meal-containing ration. The proportion of particles in the rumen DM smaller than 1 mm was 64% at 4 and 8 h and increased to about 70% at 12 h after feeding both rations. The critical particle size was found to be 0.52 mm, rumen outflow rate of the solid fraction was 6.6% and liquid fraction 15.5%/h; these parameters were not affected by the protein source in the diet.