Abstract: To understand the regulation mechanism of rumen-protected vitamin D on dairy cows' hypocalcaemia in transition period, in the experiment, forty cows from an intensive dairy farm were divided randomly into four groups (the control group (C), Ⅰ, Ⅱ, and Ⅲ groups, 10 cows per group) according to the different dosage of rumen-protected vitmin D (0, 1.5, 3.0 and 6.0 g per day for each cow) supplemented into diet from day 10 before calving to parturition. And plasma biochemical and endocrinic parameters (1, 25-(OH)₂D₃, Ca, hypocalcaemia (HYP), parathyroid hormone (PTH) and calcitonin (CT))were measured at 10, 7, and 3 d before calving parturition, at parturition,and at day 7, 14, and 21 postpartum. Results showed that level of plasma Ca was lowest at parturition, and increased gradually after calving in all groups. Ⅰgroup had higher plasma Ca from 7 d before calving parturition to 10 d postpartum. compared with the control group (P<0.05), the concentration of 1, 25-(OH)₂D₃, HYP and CT in test Ⅰ group were higher than the control group at parturition (P<0.05), the concentration of PTH was lower than the control group (P<0.05). Conclusion indicated protective vitamin D added into diet in transition period might be concern with regulation of plasma PTH and CT, which could enhance absorption of calcium in intestinal tract, stimulate mobilization of bone calcium, and then prevent hypocalcemia at postpartum, and effect of 1.5 g dosage of rumen-protected vitamin D was the best. Dairy cows' hypocalcaemia in transition period was closely associated with blood calcium homeostatic regulating action.

Key words: dairy cows; transition period; rumen-protected vitamin D; hypocalcaemia; regulation mechanism