Danish Holstein dairy cows in late lactation and milked in the morning only were used as a model for dry pregnant cows to determine the effect of oral drenching with zeolite A and zinc oxide, respectively, on total serum calcium. Ten cows were assigned randomly to two groups of five cows each, given either synthetic zeolite A (group A) or zinc oxide (group B). Blood samples were drawn daily at 10 a.m. and 10 p.m. during the whole experiment, and total serum calcium was determined. Daily fluctuations in blood calcium were recorded, with morning values being consistently lower than evening values. Oral drenching with a single dose of zinc oxide of 100 mg/kg of body weight as well as with zeolite in doses of 500 g of zeolite/cow twice a day for 2.5 d was reflected in serum calcium levels. In the group given zeolite A, there was a depression in evening values of total serum calcium although the difference did not reach statistical significance. It was followed by an increase above baseline level ("overshooting"). This was interpreted as a response from the calcium homeostatic mechanisms. In the group given a single dose of zinc oxide, a decrease in total serum calcium occurred. This decrease was not followed by overshooting, indicating that the single treatment with zinc oxide did not stimulate the calcium homeostatic mechanisms. The perspective of this first attempt to reduce dry cow ration calcium availability may be seen in relation to difficulties in formulating dry cows rations from home grown forage sufficiently low in calcium to elicit a hypocalcemia protective response at calving.