Objectives of this experiment were to determine the length of exposure to an acidogenic diet (ACD) to elicit an increased response to parathyroid (PTH)-induced changes in blood Ca in prepartum cows. The hypothesis was that cows have increased PTH responsiveness within 3 d of feeding an ACD. Ten parous Holstein cows at 242 ± 7 d of gestation were blocked by lactation (1 or > 1) and pretreatment DMI and, within block, they were assigned randomly to an alkalogenic (ALKD; DCAD = +209 mEq/kg DM; n = 5) or an ACD (DCAD = −168 mEq/kg DM; n = 5) on experiment d 0. Water and DMI were measured and blood sampled daily. Urine was sampled every 3 h for 36 h and then daily. The PTH challenges were performed on d 3, 8, and 13. Cows received 0.05 mg PTH/kg BW i.v. every 20 min for 9 h to mimic the pulsatile release of endogenous PTH. Jugular blood was sampled at 0 h, and hourly thereafter until 10 h, and at 12, 18, 24, 36, and 48 h relative to the challenge. Blood acid-base measures and concentrations of ionized Ca (iCa) were evaluated. Results were available for the first challenge on d 3 and data were analyzed by ANOVA with mixed models with SAS. Cows fed ACD had lower (P < 0.01) blood pH (7.382 vs. 7.429 ± 0.005), base excess (−2.4 vs. 4.3 ± 0.5 mM), and bicarbonate (22.8 vs. 28.5 ± 0.4 mM) within 24 h of the experiment compared with cows fed ALKD. Urine pH decreased (P < 0.01) by 15 h of feeding ACD (7.32 vs. 8.18 ± 0.17), and differences increased by 24 h (6.46 vs. 8.10 ± 0.17). Blood iCa increased (P < 0.01) in ACD compared with ALKD by d3 (1.28 vs. 1.22 ± 0.01 mM). During the PTH challenge on d3, cows fed ACD had a higher (P < 0.01) concentration of blood iCa than cows fed ALKD (1.42 vs. 1.33 ± 0.01 mM). Nevertheless, the increment in iCa in the first 36 h after the challenge, relative to baseline at 0 h, did not differ between treatments (ACD = 0.16 vs. ALKD = 0.15 ± 0.01 mM). Diet-induced metabolic acidosis occurred within 24 h of treatment; however, an increase in blood iCa concentration was observed after 3 d of metabolic acidosis. Blood iCa response to a PTH challenge did not differ between treatments on experiment d 3.