The goal of this research was to characterize ruminal pH changes that occur pre- and post-weaning (until wk 12 of life) in calves fed an elevated plane of nutrition pre- and post-weaning. Six Holstein dairy bull calves (45 ± 1.5 kg birth weight) were fed milk replacer (MR; 150 g/L) at 15% of body weight in 2 equal volumes per day. Milk volumes were adjusted weekly according to body weight. Calves were weaned using a 1 step-down weaning method; therefore, at the end of wk 5 MR was decreased to 50% and at the end of wk 6 calves were weaned. All 6 calves were fitted with ruminal cannulas in the second week of life. Calf starter, chopped straw and water were offered ad libitum and recorded daily while body weight was recorded weekly. Ruminal pH was measured continuously every 5 min from wk 5 to 12 with a ruminal pH bolus. Data were analyzed using MIXED procedure of SAS. The model included week as a fixed effect and calf as a random effect. Body weight increased from 45 ± 1.5 kg at birth to 109 ± 3.0 kg in 12 weeks, while body weight was maintained at 79 ± 1.5 kg and 77 ± 2.7 kg during step-down (wk 6) and the week after weaning (wk 7) respectively. During step-down, starter intake increased significantly (P < 0.01) from 130 ± 26.7 g/d (wk 5) to 385 ± 41.8 g/d (wk 6) and kept increasing as calves aged (2824 ± 66.9 g/d; wk 12). Pre-weaning the average daily pH was low (5.6 ± 0.09), implying ruminal acidosis. The pH reached its lowest levels during the week after weaning (wk 7) with a mean of 5.5 ± 0.11 and did not increase before wk 11. Furthermore, ruminal pH was below 5.5 and 5.2 for approximately 745 ± 187 and 220 ± 78 min daily during wk 7 and 8, respectively. The pH increased significantly in wk 11 and 12 (P ≤ 0.01), with a mean pH of 6.1 ± 0.12. In conclusion, our results show that even before step-down weaning, ruminal pH is depressed although starter intake is low. Furthermore, it demonstrates that dietary rumen adaptation post-weaning can take several weeks because of prolonged ruminal pH depression in calves fed an elevated plane of nutrient post-weaning. The prolonged depressed ruminal pH might impact gut development as well as gut health.