1. Peak 47Ca absorption and 7 day 47Ca retention were measured by a whole-body radioactivity counting technique in 10 haemodialysis patients before and after treatment with 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] and 24,25-dihydroxycholecalciferol [24,25-(OH)2D3].
2. Before treatment all patients had low peak 47Ca absorption and 7 day 47Ca retention.
3. After treatment with 1,25-(OH)2D3 (0.25-1 μg/day for 4–12 months) peak 47Ca absorption and 7 day 47Ca retention returned to normal.
4. After treatment with 24,25-(OH)2D3 (2 μg/day for 4–12 months) peak 47Ca absorption and 7 day 47Ca retention remained at pre-treatment levels.
5. It is concluded that physiological doses of 24,25-(OH)2D3 have no effect on calcium absorption or retention in uraemic man.