Three trials evaluated effects of supplemental protein and naloxone injections on intake, digestibility and digesta kinetics in lambs fed oat hay or barley straw. Daily straw dry matter (DM) and water intakes were increased (P<0.05) by protein supplementation (Trial 1). No differences were observed as a result of naloxone injections. Oat hay DM intake increased linearly (P<0.01) with protein level (Trial 2; average 22.1 g/kg body weight (BW) for supplemented lambs vs. 16.0 g/kg BW for controls). Ruminal fluid dilution rate was not affected by supplementation, but fluid volume was greater (P<0.05) in lambs supplemented at 100% of requirement (5.9 liters) than in lambs fed 125 or 150% of requirement (average 4.8 liters) or controls (4.6 liters). Supplementation increased particulate passage rate linearly (P<0.03), and decreased mean gastrointestinal and ruminal retention times linearly (P<0.03) in supplemented lambs. After 48 h of incubation, in situ disappearance of DM and neutral detergent fiber (NDF) were increased linearly (P<0.04) by supplemental protein. Total tract DM and NDF digestion were not affected by supplemental protein (Trial 3), but N retention increased linearly (P<0.05) as level of supplement increased. Supplemental protein beyond 100% of requirements had little additional effect on hay intake but increased N retention. Effects of supplemental protein on intake of low-quality roughages by sheep appear to be mediated primarily through altered digestion kinetics rather than metabolic events.