Adjuvant polyarthritis (AP) in rats is known to result in extensive bone loss. This study investigates the mechanisms responsible for the early trabecular osteopenia evaluated at a single point in time--2 weeks after adjuvant injection--in the hindpaw of female Lewis rats using biochemical and histomorphometric methods. At this early point in time, the inflammation was generalized (inflammatory score, 20; albumin/globulin, -80% versus control). Histomorphometric analysis of the noninjected femur showed that the trabecular bone volume was significantly decreased (-28% versus control) in both proximal and distal parts, and the femur growth rate was unaffected. The trabecular osteopenia was associated with a 90% decrease in osteoid surface and a concomitant thinning (-19%) of the trabeculae. Both the double-fluorescence-labeled surface and the osteoblast surface were also markedly decreased (-75%). In addition, the mineral apposition rate was reduced (-50%) and the bone formation rate was decreased by as much as 90%. The trabecular bone volume was decreased in relation with the extent of double-fluorescence labeling (r = 0.38, p = 0.03) and bone formation rate (r = 0.42, p = 0.01), suggesting that the generalized osteopenia resulted from the reduced bone formation. This was associated with a 26% reduction in plasma osteocalcin. Neither the osteoclast surface nor the number of osteoclasts was consistently affected. However, urinary hydroxyproline was increased by 100-200%, which likely reflected the cartilage and bone destruction at the site of injection. The present data show that the early extensive osteopenia observed 2 weeks after AP induction in rats results from defective bone formation with unchanged bone resorption. The role of cytokines in such an inhibitory effect on bone formation remains to be determined.