The rat remnant kidney, a popular model of experimental renal failure, has also been used to assess the histological bone changes associated with reduction in renal mass. The suitability of this model has been challenged, especially with regard to the standardization of animals from different age groups and various degrees of renal failure. The present study was undertaken: (1) to better evaluate the histomorphometric changes associated with reduction in renal mass; (2) to assess these changes over a longer follow-up period as compared to those of matched intact animals; (3) to define the response to pharmacological doses of calcitriol. Male rats were evaluated 4 and 8 weeks after induction of renal failure (5/6 nephrectomy) and compared with age-matched control rats with intact kidneys. Plasma biochemistry, creatinine clearance and parathyroid hormone (PTH) were obtained from normal rats and from the rats with chronic renal insufficiency. Histomorphometric studies were performed in all animals on the right tibial bone. Induction of renal failure of 4 weeks' duration (short-term study, age 16 weeks) was associated with increased PTH and bone resorption, but suppressed bone formation. At long-term follow-up (rats aged 20 weeks), the suppression in bone formation was even more prominent. These changes were assessed considering normal bone histomorphometry during the process of growing. Calcitriol administration was associated with a significant suppression of bone resorption and a spectacular increase in all osteoid parameters. The bone formation rate, however, remained in the same low range as that in untreated animals. This model of renal osteodystrophy is similar to human secondary hyperparathyroidism with regard to the enhanced bone resorption, but differs by the marked decrease in bone formation. There was a remarkable similarity between the response of rats with renal failure and that of patients with severe secondary hyperparathyroidism to large doses of calcitriol, leading in both to adynamic bone disease. Keeping in mind the species differences, the bone changes developing in the uraemic rats may be useful in understanding certain aspects of human renal osteodystrophy.