The effect of 1 - 3 intramuscular injections of the vitamin D3 metabolite 25-OHD3, synthetic 1 alpha-OHD3, and a combination of both on the plasma concentrations of calcium (Ca), inorganic phosphate (P(i)) and magnesium (Mg) was tested in 67 older dry dairy cows. Administering 4,0 mg 25-OHD3 reduced Ca and P(i) for some time while Mg blood levels remained constant. Injecting 210 micrograms 1 alpha-OHD3 plus 2.0 mg 25-OHD3 only brought a marked increase in P(i) concentrations, whereas 420 micrograms 1 alpha-OHD3 plus 4.0 mg 25-OHD3, and 350 micrograms 1 alpha-OHD3 alone significantly raised Ca and P(i) levels beginning on the 2nd day after injection and lasting for at least 6 days. These higher levels can be maintained by repeated injections. Another effect of injecting 1 alpha-OHD3 was to depress Mg blood levels, and a negative effect on the Ca and vitamin D3 metabolism is suggested which may cause hypocalcaemia when the effect of 1 alpha-OHD3 subsides. Metastatic calcifications of soft tissues were observed in some 1 alpha-OHD3-treated cows.