In a former study we have shown that 1 alpha OH vitamin D3 given to hemodialyzed patients taking A1(OH)3 at a constant dose increased their plasma concentrations of aluminium. Two mechanisms can explain this increase: increased intestinal absorption or decreased tissue storage of aluminium. We have, in the present study, given 1 alpha(OH)3 at the same dose (6 micrograms per week) and during the same duration (4 weeks) to 15 stable hemodialyzed patients after aluminium hydroxide has been discontinued 6 weeks before. Under A1(OH)3 treatment mean plasma aluminium was 2.33 +/- 2.36 mumol/l. After A1(OH)3 discontinuation, plasma aluminium decreased significantly as soon as the 2nd week of the control period (1.39 mumol/l). The decrease was maintained in plateau throughout the 5 weeks of the control period (1.38 mumol/l and, the 4 weeks of 1 OH Vit D3 administration (1.40 mumol/l). After 1 alpha OH D3 discontinuation there has been a non significant transient drop of aluminemia followed by a plateau at a level comparable to the previous plateau. Plasma calcium and phosphate concentrations increased significantly with 1 alpha(OH)3 and decreased thereafter confirming biological activity of 1 alpha(OH)3. Since 1 alpha(OH)3 increases plasma aluminium in hemodialyzed patients only when they are taking simultaneously A1(OH)3, it is suggested that this increase is mainly explained by an increase of the intestinal absorption of aluminium.