We determined the quantitative relationships between graded oral dosing with vitamin D₃, 25(OH)D₃, and 1,25(OH)₂D₃ for short treatment periods and changes in circulating levels of these substances. The subjects were 116 healthy men (mean age, 28 + 4 years, with usual milk consumption of 40.47 l/day and mean serum 25(OH)D of 67 + 25 nmol/l). They were distributed among nine open-label treatment groups: vitamin D₃ (25, 250 or 1250 mg/day for 8 weeks), 25(OH)D₃ (10, 20 or 50 mg/day for 4 weeks) and 1,25(OH)₂D₃ (0.5, 1.0 or 1.0 mg/day for 2 weeks). All treatment occurred between January 3 and April 3. We measured fasting serum calcium, parathyroid hormone, vitamin D₃, 25(OH)D and 1,25(OH)₂D immediately before and after treatment. In the three groups treated with vitamin D₃, mean values for circulating vitamin D₃ increased by 13, 137 and 883 nmol/l and serum 25(OH)D increased by 29, 146 and 643 nmol/l for the three dosage groups, respectively. Treatment with 25(OH)D₃ increased circulating 25(OH)D by 40, 76 and 206 nmol/l, respectively. Neither compound changed serum 1,25(OH)₂D levels. However, treatment with 1,25(OH)₂D₃ increased circulating 1,25(OH)₂D by 10, 46 and 60 pmol/l, respectively. Slopes calculated from these data allow the following estimates of mean treatment effects for typical dosage units in healthy 70-kg adults: an 8-week course of vitamin D₃ at 10 mg/day (400 IU/day) would raise serum vitamin D by 9 nmol/l and serum 25(OH)D by 11 nmol/l; a 4-week course of 25(OH)D₃ at 20 mg/day would raise serum 25(OH)D by 94 nmol/l; and a 2-week course of 1,25(OH)₂D₃ at 0.5 mg/day would raise serum 1,25(OH)₂D by 17 pmol/l.