Glucocorticoids at pharmacological concentrations cause osteoporosis and aseptic necrosis, particularly in the proximal femur. Several mechanisms have been proposed, but the primary events are not clear. We studied changes in the bone structure and cellular activity in femora of glucocorticoid-treated rabbits before the occurrence of fracture or collapse. In rabbits treated 28 days with 4 micromol/kg.day of methylprednisolone acetate, changes in the cortical bone were minor. However, metabolic labeling showed that bone formation was virtually absent in the subarticular trabecular bone, and scanning electron microscopy showed resorption of 50-80% of the trabecular surface. Thus, reduction in bone synthesis and increased resorption were involved in bone loss. Vascular changes, which have been hypothesized to mediate glucocorticoid damage, were not seen, but histological changes suggested that trabecular bone was damaged. Matrix integrity was examined using laser scanning confocal microscopy to detect passive tetracycline adsorption. In treated animals, but not controls, tetracycline was adsorbed, in a novel lamellar pattern, in 50--200 microm regions extending deep into trabeculae. This showed that the matrix, which is normally impervious, was exposed at these sites. TUNEL assays showed that matrix damage correlated with cell death in the subarticular trabecular bone of treated animals. The pattern of cell death involving cohorts of osteoblasts and osteocytes comprised up to half of the bone volume in affected regions and is consistent with an apoptotic mechanism. Small numbers of TUNEL-labeled osteoblasts, but no osteocytes, were detected in control bone. We conclude that exposure of bone matrix permeability and that regional cell death consistent with apoptosis is an early event in glucocorticoid-induced bone damage.