incremental levels of rumen undegradable protein (RUP) on nutrient utilization of growing steers fed high fiber hay. Metabolism study: Holstein steers (n = 6, 217 13.8 kg) were used in a replicated 3 x 3 Latin Square designed experiment. Incremental levels of RUP (32.5, 40.13, and 42.48% of CP) were fed as supplements at 1.72 kghd -1 d -1 (AF) to a bermudagrass (Cynodon dactylon L.) hay diet (10.2% CP, 76.4% NDF). Supplements were iso-nitrogenous (25.9% CP) and fed as pressed molasses blocks. The RUP was controlled by feeding different ratios of SBM, corn gluten feed, poultry protein meal and soy hulls. Corn was added to the diets to increase energy levels. Steers were fed treatments for 14 days. Steers were placed in metabolism crates for total fecal and urine collection from d 10 to d 14. Jugular blood was sampled before and after feeding on day 10 and 14, respectively. Hay and total DMI increased (3.95 to 4.27 kg/d and 5.56 to 5.92 kg/d) with RUP (P < 0.05). Fiber digestion was larger (P < 0.05) when steers were fed the high RUP diets due to inclusion of soy hulls; however DMD, OMD and DE were not affected (P > 0.10). Urinary N output was lower (P < 0.05; 23.82 1.45 g/d) in steers fed the high RUP diets but treatments did not affect N retention (P > 0.10; 49.46 2.13 g/d). Blood urea nitrogen (BUN) increased after feeding (P < 0.05; 14.15 vs. 16.3 mg/ dl) and tended to be negatively related to RUP. Performance study: British and Brahman x British steers (n = 48; 252.15 22.5 kg initial BW; age 9 mo.) were ranked by BW, randomly allocated to one of 8 pens and fed supplement in two forms (pressed blocks or meal) and two levels of RUP (27 vs. 44% of CP for blocks and 30 vs. 46% of CP for the meal) in a 63-d 2 x 2 factorial feedlot trial. Bermudagrass hay (13.1% CP, 74.59% NDF) was offered free choice and supplements were fed at 1.72 kghd -1 d -1 . Blood samples were collected on d 21 and d 63. Total DMI was not affected by treatments. Steer ADG (0.67 0.05 kg) did not respond to RUP or form of supplement. Diet evaluation indicated that ME allowable gain was lower than the MP allowable gain. Blood urea nitrogen was lowered with RUP (14.1 vs. 12.1 mg/ dl; P < 0.01) and at d 21 than at d 63 of the experiment. An interaction between RUP levels and form was observed for blood glucose and insulin (P < 0.01). Despite some indications that RUP improved nutrient utilization, animal performance was not affected, mainly because of low range of RUP consumed among treatments and because of levels of protein consumed related to energy.