Female mature eels (300 to 500 g) received one intraperitoneal injection of 1.25(OH)2D3 (10 microgram). Their vertebral bone was compared, 8 h and 24 h after the injection, with vertebral bone of control mature female eels receiving solvent alone (ethanol). Sexual maturation in female eels induces a bone decalcification with hypercalcaemia and hyperphosphataemia. The control eels showed marked osteoclastic resorption and osteocytic osteolysis and the degree of mineralization of the intercellular substance decreased. Injection of 1.25 (OH)2D3 into these female mature eels provoked as early as 8 h: 1) an increase in hypercalcaemia and hyperphosphataemia; 2) a major conversion of lining cells to osteoblasts and a stimulation of osteoblastic activity with new bone formation; 3) diminished osteoclastic resorption without changing osteocytic osteolysis or bone matrix mineralization.