An artificial mastication and a sieving analysis was developed to test the theory that the rate of forage consumption by ruminants is proportional to the rate at which the forages are reduced to particles small enough to pass the reticulo-omasal orifice and thus will be ejected from the rumen. The masticator was constructed from a double-gear pump to allow a slurry of 10 g of forage in 2.6 1 of water to be continually recycled through the gear aggregate. This provided the mastication process. After mastication, the forage was dried and a standard sieving analyses was employed to determine a particle size index. A low error among indexes from replicated analyses, and significant differences between forages proved that the procedure was repeatable and sensitive. Artificial mastication of 14 samples of hay that had been fed to sheep, showed that voluntary feed intake (Y) and particle size index (X) were significantly (P < 0.01) correlated (r = 0.94). The regression of Y on X (Y = 353X - 14) was significant at P < 0.001 and estimated the daily dry matter consumption by a 100 pound sheep to an accuracy of [plus or minus] 127 gm (P < 0.05) (the standard error of estimated Y multiplied by Student''s "t" at the 5% level of probability).