The objective of this study was to determine the effects of low rumen pH in dairy calves on performance, rumen passage rate and blood and rumen tissue characteristics. Starter diets were formulated to induce (A) or blunt (B) rumen acidosis in young dairy calves. Diet A was a complete pellet (42% starch, 13% NDF) and B was texturized (31% starch, 22% NDF). Ten (n = 5/diet) Holstein bull calves were assigned to treatments at birth, cannulated at wk 3, and received milk replacer and a starter diet through wk 7, then starter only through wk 16. Rumen contents were sampled and pH recorded −8, −4, 0, 2, 4, 8, 12, and 24 h relative to starter feeding during wk 6, 8, 10, 12, 14, and 16 of age. Cobalt-EDTA was used to estimate liquid passage rate and rumen volume. Blood was drawn once each sampling week to determine complete blood cell count, pH, hematocrit, hemoglobin concentration, and partial pressures of CO₂ and O₂. Calves were weighed weekly; starter intake was recorded daily. Calves were slaughtered and rumen tissue collected at wk 17. Formalin-fixed ruminal tissues were given a lesion score (0 = healthy tissue, 5 = severe degradation and inflammation). Statistical analyses were conducted using mixed models in SAS with P < 0.05 considered significant. Mean (min, max) rumen pH was lower for A than B calves: 5.4 (3.3, 7.2) and 5.6 (3.5, 6.8). Lowest mean pH values, 5.0 and 5.5 for A and B diets, were observed in wk 8. Starter consumption and BW were lower in A calves beginning at 3 and 5 wk, respectively. Carcass weights were 68.1 and 82.2 ± 1.8 kg for A and B calves. Rumen liquid volume tended to be greater in B calves (P = 0.08) and increased with age. Liquid passage rate was similar between treatments. Blood hemoglobin concentration and hematocrit were reduced in A calves. Blood pH decreased linearly with week of age. No clear trends were observed for other blood parameters. Mean rumen lesion scores of rumen tissue were 3.7 and 2.3 ± 0.2 in A and B calves. Growth and physiological responses were negatively affected in calves receiving A.