Twelve wether lambs (32kg) with abomasal and ileal cannula were fed a 90% concentrate basal diet (800 g/d), basal + 1.5% magnesium oxide (MgO)(812 g/d), basal + 1.5% limestone (812 g/d) or basal + 3.0% limestone (824 g/d) to study the effect of·these minerals on intestinal pH, rumen fermentation, N balance, dry matter and crude protein digestibility (DMD, CPD), and small intestinal disappearance of amino acids (AAD). Limestone (3%) increased (P < .03) rumen pH. Magnesium oxide increased (P < .10) ileal and fecal pH. Limestone significantly increased N absorption and pre-abomasal DMD, but decreased (P < .03) AAD. N retention was not improved by the treatments. An 82-d feeding trial was conducted with 72 wether lambs (avg initial wt: 28 kg) to study the effect of 1 or 3% fine (70% < 53 µ) or coarse (85% > 425 µ) limestone on rumen environment, weight gain and feed efficiency of lambs fed an all-concentrate diet. Rumen pH and VFA molar proportions were not affected by the treatments. Limestone (3.0%) decreased (P < .10) total rumen VFA concentrations and increased (P < .10) fecal pH. Weight gain was not different (P > .10) among the treatments. Coarse limestone increased (P < .10) feed efficiency. Five Angus heifers (285 kg) with duodenal and ileal cannulae were fed a 90% concentrate control diet (7.5 kg/d) or the same diet containing 1.60% defluorinated phosphate-regular (5.5%, 19.0% and 33.0% on 1400, 1180 and 850 µ sieves, respectively, DRP-R), 1.60% defluorinated phosphate-coarse (85% evenly among large sieves, DRP-C), 1.28% limestone or .5% MgO to study the effect of limestone or MgO on intestinal pH, DMD, starch digestibility (SD), CPD and AAD in beef cattle fed a high-concentrate diet. Ileal pH was increased by MgO. Fecal pH was increased (P < .05) as follows: MgO > DRP > limestone and control. Minerals increased (P < .05) duodenal liquid flow. Limestone and DRP-C increased (P < .05) acid flow to the duodenum. Total tract DMD, SD and CPD were similar among treatments. Limestone and DRP-R increased (P < .10) AAD. DRP-C tended to increase AAD, but differences were not statistically significant. / Ph. D.