Accelerometers can be used to gauge rumination activity (RumAct). The purpose of this study was to determine if hypocalcemia affected RumAct in older periparturient cows fed a high or low DCAD diet to induce or prevent periparturient hypocalcemia. Twenty-six Holstein cows entering their 3rd or greater lactation were assigned to an Anion Supplemented (AS) or No Anion precalving diet and fed behind Calan gates. After calving, all cows were fed the same lactation diet. DMI were determined for each cow from 14 d before calving until d 5 of lactation. Blood was sampled daily the wk before calving, at calving, and at 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4 and 10 d after calving. Urine sample pH determined before calving was significantly decreased in cows fed the AS diet (6.99 + 0.16 vs. 8.26 + 0.06). The effects of diet, d around calving and their interaction on plasma Ca, DMI, and RumAct were assessed by the Mixed Procedure of SAS, with d around calving as the repeated measure. Cows fed AS diet had significantly higher blood Ca concentrations from 24 h before calving thru the first 36 h of lactation (P < 0.05). Four No Anion cows developed milk fever necessitating IV Ca treatment. On the 1st and 2nd d of lactation, cows fed AS diet consumed significantly more DM (3.85 kg on d 1, P < 0.01; 3.39 kg on d 2, P < 0.025) than cows fed the No Anion diet. RumAct decreased in all cows at parturition. Diet had no effect on 24 h RumAct until the day before calving. During that 24-h period, cows fed AS spent 457 m ruminating while No Anion cows spent only 356 m ruminating; a 22% decrease (P = 0.005). RumAct was greater in cows fed anions during the 1st and 2nd 24 h periods of lactation with anion fed cows ruminating 127 and 115 more min/d during the 1st and 2nd 24 h periods after calving respectively (P < 0.002). RumAct data were also compiled into min of RumAct per 2 h period. Cows fed AS had significantly greater RumAct (P < 0.05) than No Anion cows during 13 of the 24 2-h periods between 12 h before calving thru the first 36 h after calving. Milk fever cows had extended periods where the rumination rate / 2 h was undetectable, even after treatment restored blood Ca concentrations to normal levels.