I . Changes in strength of cotton, Italian ryegrass leaf, hay stem and barley-straw stem on immersion in the contents of the reticulo-rumen, at two sites in the rumen and at one site in the reticulum, for periods of up to 56 h werc measured using a cow on a hay diet. Specimens of the fibrous materials were immersed in the rumen digesta in such a way that they were not damaged by agitation or by rumination.z. Load-to-fracture and elongatiun were measured on a machine using a constant-speed cross-head at a strain of about I O -~/ S . The ratio strength: dcnsity (hreaking loadlmass per unit length) was determined.3. Leaf of Italian ryegrass and stem of hay and of barley straw had strengths of about 35, ijo and 210 lMN/m2. They lost strength at different rates when immersed in the rumen digesta, the times to half strength being in the ratio grass: hay: straw = I : 1'9: 3.8. The similarity of this ratio to that for total time for eating and ruminating as givcn by Balch (1969), namely 1:z.o: 3'5, for similar materials was noted.Differences in the physical properties of fibrous foodstuffs and changes that such properties undergo in the reticulo-rumen are thought to affect intake and digestion. The usual approach is to measure voluntary intakes for foodstuffs of different chemical composition, the proportion of dry matter that is present as cell-wall constituent being generally held to be a significant factor affecting both intake and digestion (Colburn, Evans & Ramage, 1968; Welch & Smith, 1969a, 6; Cammell & Osbourn, It has been shown (Evans, 1964) that there is a correlation between strength and cellulose content in ryegrass and (Evans, 1967) that strength depends on position of leaf on the plant, on age of the plant, and on the position of the specimen along any one leaf. Smith, Goering, Waldo & Gordon (1971) found that the rate of digcstion of cell wall was negatively correlated with the ratios lignin: acid-detergent fibre (Y = -0.60) and 1ignin:cellulose ( r = -0.57) and positively correlated with content of soluble material in the cell dry matter (DM) (Y = + 0-77).