Osteosarcoma-derived osteoblastic cells were exposed to 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) either to 0.001-1,000 nmol.l-1 for 4 days or to 10 nmol.-1 for 1-21 days. Between 0.01 and 10 nmol.l-1, a dose-dependent increase in alkaline phosphatase (AP) activity was found, which rose to a maximum level at 10 nmol.l-1 (+55%). At higher doses (100 and 1,000 nmol.l-1), 1,25(OH)2D3 induced a decrease in AP activity of 40%. After 1 day at 10 nmol.l-1 a slight increase in AP activity was noted (+20%) which augmented with the duration of exposure. This stimulatory effect was highest after 8 days (+130%). In contrast, after 15 and 21 days, AP activity decreased by 30%. Under 1,25(OH)2D3, microtubules were observed mainly in the perinuclear region.