In five trials carried out on two lactating and two dry cows three sets of particle sizes were used: in the first set particles with a diameter of 9 mm and length of 4, 6, 8 and 10 mm, in the second set particle sizes of 8/8,11/11,14/14 and 17/17 (diameter/length) mm, and the third set 8/8,9/9,10/10,11/ 11 and 12/12 mm particles. In the first set the lactating cows had a significantly higher mean recovery (97.8 vs 75.9 %) and a significantly shorter total tract mean retention time (TMRT) (53.0 vs 96.6 h) than the dry cows (P<0.05). In the second trial with lactating cows the main differences in recovery were found between 11/11 and 14/14 mm particles, significant at 48 h after administration (63.8 vs 31.3 %; P<0.05). Regurgitation of 17/17 mm particles was 27.5 % in lactating cows and 80% in dry cows in the course of 384 h. In the third set did not yield significant results, except 36 h (P<0.05) after placing. It is concluded that relatively good recovery and TMRT can be achieved in lactating cows with plastic particles of a size larger than 11/11 mm but smaller than 14/14 mm.