Obesity is associated with altered bone mass. However, reports on bone status in obesity are inconsistent. Increased or normal bone mass was reported in obese adults but decreased bone mineral content was described in obese children. Therefore we evaluated the obese fa/fa rat as a possible model to assist in studies of bone metabolism in obesity. Obese and lean 14-week-old male rats underwent 24 h balance studies for calcium, magnesium and phosphate. Plasma calcium, magnesium, phosphate, immunoreactive parathyroid hormone, urinary cAMP (cyclic adenosine monophosphate) and femur bone histomorphometry were also analysed. Obese rats were heavier and had higher plasma insulin, cholesterol and triglycerides levels (P less than 0.05). A comparable positive balance for calcium, magnesium and phosphate was found in obese and lean rats. Total plasma calcium was higher in the obese, but albumin corrected calcium and plasma magnesium, phosphate and glucose were similar to the lean. In contrast to human obesity, obese rats were hypercalciuric, hypermagnisuric and hyperphosphaturic (P less than 0.05). iPTH and urinary cAMP were higher in the obese. Femora of fa/fa rats were shorter and lighter. Their bone osteoid surface and bone calcium content were similar to controls. Femora metaphysis in the obese had increased number of trabeculae, decreased trabecular width and higher erosion surface/bone surface ratio. Their diaphysis had increased cortical area/bone area and cortical width/bone width ratios and decreased medullary area. In summary, obese rats have higher iPTH, are hypercalciuric and have decreased bone mass. These last two observations differ from what is described in adult human obesity. Therefore, the obese fa/fa rat is of limited assistance in studies of bone status in adult human obesity. It might be of help in studies of bone metabolism in juvenile obesity.