In 10 patients with early renal failure (glomerular filtration rate between 30 and 80 ml/min/1.73 m2) without nephrotic syndrome and in 13 controls the calciuric response to increasing oral doses of calcitriol [1,25-(OH)2D3] (0.25; 0.5; 1.0; 1.5 micrograms/day) was evaluated. Patients had initially normal plasma calcitriol levels, normocalcemia and hypocalciuria. With calcitriol doses up to 1 microgram/day, the increment of UVCa was less in patients. Only at 1.5 micrograms/day the calciuria of patients reached the baseline calciuria of controls. The data document that 1.0 microgram calcitriol per day is required to normalize UVCa in patients with early renal failure and normal calcitriol levels. Blunted calciuric response to low doses of calcitriol is compatible with an altered dose-response relationship.