1. Eight wether sheep were offered a diet of perennial ryegrass (Lolium perenne cv. Endura) hay once daily at two levels of intake (11 and 16.5 g dry matter (DM)/kg live weight (LW] with or without maize starch (175 g DM/kg hay DM). The experiment consisted of four periods, each lasting 32 d. Rates of digestion of the hay were measured by incubation in dacron bags and rates of passage using chromium-mordanted hay. Rumen pool sizes of DM, organic matter and fibre were measured by emptying the rumen. 2. The inclusion of starch in the diet appeared to increase significantly (P less than 0.01) the lag phase before the start of fibre digestion, as observed both in the dacron bag studies and in the slower initial disappearance of digestible neutral-detergent fibre (NDF) from the rumen recorded by emptying of rumen contents. However, there was no significant effect of starch on apparent digestibility of fibre in the whole tract. 3. The main effect of increasing the level of feeding was to increase the rate of passage with a consequent decrease in overall digestibility. The fractional rate of passage increased from 0.0318 to 0.0400 as the level of feeding increased, while apparent digestibility of NDF decreased from 0.755 to 0.724. 4. On all treatments the weight of indigestible fibre in the rumen remained more or less constant between 5, 10 and 15 h after feeding, but was significantly lower at 24 h. These results suggest that a high proportion of the outflow of material from the rumen not associated with feeding appears to occur during the second half of the feeding cycle.