We performed two experiments using rumen-fistulated mule deer (Odocoileus hemionus hemionus) and elk (Cervus elaphus nelsoni) to test hypotheses regarding particle breakdown and passage within the rumen. In experiment 1, labeled particles of three sizes, including large particles (LP), 2.8-4.8 mm in diameter, medium particles (MP), 1-2.8 mm, and smallparticles (SP), 0.1-1 mm, were introduced into the rumen. Breakdown and passage were measured for each fraction by periodic ruminal sampling. Particle breakdown and passage exhibited first-order kinetics, but large particles were not reduced to medium particles in a simple cascade process. The process was more complex: 89% and 92% of LP in elk and deer, respectively, went directly to SP, while 84% and 100% of the MP in elk and deer were comminuted to SP before passage from the rumen. In experiment 2, five forages were introduced as LP into the rumen, and their breakdown and passage were measured. Although breakdown kinetics were significantly different betwee...