This article summarizes the results obtained in 6 separate studies concerned with the effect of zeolite A supplementation in the dry period on blood calcium, magnesium and phosphorus status around calving. The experiments were conducted on 5 different farms, and comprised a total of 117 cows. Two of the experiments (exp. 5 and 6) were conducted under extensive farming conditions whereas the rest (exp. 1-4) were conducted on intensively driven farms. All cows included in the experiments had completed at least 2 lactations. The cows were allocated as either untreated control cows or zeolite treated experimental cows according to expected date of calving and parity. The experimental cows were fed between 0.5 and 1.0 kg of zeolite A per day during the last 2 to 4 weeks of the dry period. Blood samples were drawn on the day of calving and day one and two after calving (all experiments), three weeks before the expected date of calving (exp. 1 and 2) and one week after calving (exp. 3 and 4). The zeolite supplementation significantly increased the mean serum Ca level on the day of calving. The efficiency of the zeolite supplementation to prevent hypocalcaemia (serum Ca < 2.00 mmol/l) on the day of calving was calculated. Efficiencies varied ranging from 3 to 100%, with a mean efficiency of 58%. The zeolite-calcium ratio (g of zeolite per cow per day/g of dietary calcium per cow per day) was calculated in each experiment. From the results it seemed, that zeolite-calcium ratios below 5 did not effectively prevent parturient hypocalcaemia, whereas ratios of 10 to 20 proved very efficient in preventing hypocalcaemia. There was apparently no additional effect from feeding zeolite for 4 instead of 2 weeks prepartum. Feeding zeolite in the dry period significantly decreased plasma phosphate before as well as after calving. The phosphate level was normalized within one week after calving. Plasma magnesium was significantly lower among the experimental cows on the day of calving, but stayed within the normal range of plasma magnesium. The control cows on the other hand experienced hypermagnesemia after calving.