Abstract The intake of both untreated and Ca(OH) 2-treated wheat straw was determined during a 30-day period prior to the experiment. During the experiment, which lasted another 19 days, measurements were made of the amounts and composition of digesta in the reticulo-rumen (ReRu) and flowing through the abomasum of sheep (4 per treatment) offered the untreated straw-based diet at 80% of previously measured voluntary intake (Treatment A) or the treated straw-based diet at 90, 75 or 50% of intake (Treatments B, C and D). Digestion rates of neutral detergent fibre (NDF) in and outflow rates from the ReRu were also estimated. There were minor differences in total or digestible dry matter (DM) intakes (730 vs. 650 g day −1 and 376 vs. 345 g day −1) between sheep given Treatments B and C, as some of those receiving Treatment B did not consume all of the feed offered. However, the sheep on these treatments consumed more ( P < 0.05) total and digestible DM than those given Treatments A (416 and 223 g day −1) and D (440 and 232 g day −1). The intakes of other dietary constituents were also greater for Treatments B and C than for Treatments A and D. The amount of DM accommodated in the ReRu was positively ( P < 0.05) related to DM intake, although differences in intake only accounted for about 40% of the variation in ReRu DM loads. While there were no significant differences in the amounts of total digesta and of digesta constituents in the ReRu of sheep on the different treatments, these tended to be higher on Treatments B and C. The fractional digestion rates of potentially digestible NDF (5.3 vs. 4.0% h −1) and of total NDF (3.2 vs. 1.7% h −1) in the ReRu tended to be higher in sheep given Ca(OH) 2-treated straw, although the differences between treatments were not significant. DM flow through the abomasum was positively ( P < 0.01) related to DM intake, the relationship accounting for 98% of the variation in flow. Flows of digesta constituents were greater ( P < 0.01) in sheep on Treatments B and C compared with Treatments A and D. However, fractional outflow rates of NDF from the ReRu were not different between treatments owing in part to the tendency for increased amounts accommodated in the ReRu at higher intakes. The amounts of crude protein apparently digested in the intestines on Treatments B and C were higher (29 and 27 vs. 18 and 21 g day −1) than on Treatments A and D.