The effect of oat hay, fed in the long condition and after grinding through 1/4-inch or 3/32-inch Hammermill screens, on the rumen motility of steers was studied. In one experiment all bedding was removed from the steers when the 3/32-inch hay was fed. Significant differences (P < .01) in rumen contraction amplitude existed between the steers on all the diets except between those on the two 3/32-inch hay diets. The difference in the rumen contraction frequency of the steers on diets of long and 1/4-inch hay was not statistically significant; however, both of these diets produced rumen contractions significantly (P < .05) more rapid than those for the steers on the 3/32-inch hay diets.The average time required for the rumen contraction amplitude to become uniform when the diet was changed from long hay to 3/32-inch hay was 4 wk; 1/4-inch hay to 3/32-inch hay, 5 wk; 3/32-inch hay to long hay, 6 wk; and when the sawdust bedding was removed from the calves on the 3/32-inch hay, 2 wk.Frothy rumen ingesta and frequent bloating were observed in steers receiving the 3/32-inch hay diets; one death occurred. Scabrous materials in the diet may maintain or reinforce the tonic activity of the motor neurons responsible for rumen motility.