Abstract Eighteen Angora goats (average BW 14 kg) and 18 Barbados Blackbelly X Dorset sheep (average BW 24 kg), all wethers, were fed chopped coastal bermudagrass hay (84% neutral detergent fiber (NDF), 6% crude protein (CP)) ad libitum and offered only alfalfa hay or cottonseed meal as protein supplement during an 84-day feeding trial. From day 28, animals received daily injections of physiological saline with or without the parasympathomimetric slaframine (SF) at 30 μg kg −1 BW. Digestibility and digesta kinetics measurements were made during a 7-day fecal collection period. Goats had lower ( P< 0.05) intakes of DM, NDF and CP than sheep. There were no differences in digestibility of DM, NDF or CO between species. Protein supplementation increased digestibility of DM ( P< 0.05), NDF ( P< 0.10), and CP ( P< 0.01) without affecting intake. No differences were due to type of supplement in intake and digestibility. SF depressed ( P< 0.05) intake of DM, NDF and CP and tended ( P<0.10) to lower DM and CP digestibility. Protein supplementation increased ( P<0.05) and SF decreased ( P< 0.01) weight gain. Animals receiving protein supplement lost less body weight (BW) after SF treatment than those receiving no supplement. Goats and sheep apparently utilized low quality roughages differently but responded similarly to protein supplementation. SF was not effective in altering ruminal liquid dilution rate or enhancing nutrient digestibility in goats and sheep fed high fiber diets.