This study evaluated the effects of maize and lucerne particle size on ruminal passage rate and chewing activity in midlactation cows. Four multiparous Holstein cows with ruminal cannulas were assigned randomly to a 4 × 4 Latin square design. Treatments were arranged in a 2 × 2 factorial design; two lucerne particle sizes: 2.54 mm (short) and 6.22 mm (long) were combined with concentrates based on either ground maize (711 µm) or cracked maize (1755 µm). Diets were fed as a total mixed ration with a ratio of concentrate to forage of 39:61. No significant difference in dry matter intake, ruminal passage rate and chewing activity between ground maize and cracked maize were observed. Ruminal liquid passage rate increased from 11.3 to 12.1%/h (P=0.02) and ruminal solids passage rate tended to decrease from 6.4 and 6.0%/h when short lucerne was fed to cows. Cows fed long lucerne hay spent more time ruminating compared to cows fed short lucerne hay (P<0.001). Total time spent on chewing by cows increased from 505 to 574 min/d (P=0.002) for short lucerne and long lucerne, respectively