The influence of oral calcium ± cholecalciferol or calcitriol on S-PTH and whole blood ionized calcium (B-Ca⁺⁺) in the very short term has not been elucidated. B-Ca⁺⁺ and S-PTH were measured after overnight fast every 5 or 15 min for 4-h in 7 healthy premenopausal women (30-45 years) in a crossover design where the subjects were studied on 4 different days. Study 1: (control), 125-ml tap water (also given in studies 2-4); study 2: 1000 mg calcium, as calcium carbonate; study 3: 1000mg calcium and 400 IU (5 μg) cholecalciferol; and study 4: 0.5 μg calcitriol plus 1000mg calcium. Calcium plus 1,25(OH)₂ vitamin D3 induced a rapid and significant fall in S-PTH compared to the control period (p<0.02). Calcium alone or calcium plus cholecalciferol did not change the PTH levels significantly compared to control (water). B-Ca⁺⁺ increased significantly after calcium plus 1,25(OH)₂ vitamin D₃ (p<0.01) and calcium plus cholecalciferol (p<0.05) compared to the control period. The B-Ca⁺⁺ elevation was significantly higher after calcium plus 1,25 vitamin D₃ than after calcium plus cholecalciferol (p<0.05). In conclusion, oral calcitriol plus calcium causes a rapid elevation in B-Ca⁺⁺ and suppression of the PTH secretion also in the short term (hours).