This is a comparative study of massive single dose treatment (400 000 I. U. vitamin D3, divided in 2 X 200 000 I. U. on day 1 and day 3, orally) versus continuous treatment (9600 I. U. vitamin D3-drops for 18 days) of vitamin D-deficiency rickets (VDR). To estimate the effectiveness of these two kinds of treatment the serum concentrations of calcium (Ca), phosphorus (P), alkaline phosphatase (aP), calcitriol (25-OH-CC), parathyroid hormone (PTH) and calcitonin (Ct) before and during treatment (on day 3, 7, 14, 21) were studied in 10 patients, 5 in each group, with VDR. There was no difference in the course of serum concentrations of Ca, P and aP in these two treatment-groups. The elevated PTH returned more quickly to normal range in the single dose study group. Ct-serum concentrations showed too different courses in the studied patients, therefore no comparison could be made. 25-OH-CC showed a very quick increase in the single dose-group and a slower one in the continuous treatment group the climax in both groups reached a value far over the tentative range. According to these results an unequivocal better effect of one of these two compared VDR-treatments could not be seen, what means that both treatments can be recommended.