Bone morphology and function were studied in male spontaneously diabetic BB rats after 3-4 weeks of diabetes. The tibia and lumbar vertebrae weights were decreased, but the bone calcium percentage remained normal. Bone volumes in the tibial metaphysis and the first lumbar vertebra were normal on quantitative histomorphometry. Osteoclast, osteoblast, and osteoid surface percentages, however, and the calculated daily mineral apposition rate in the tibia (1.0 +/- 0.4 vs. 5.6 +/- 0.6 microns/day) and vertebra (0.2 +/- 0.1 vs. 2.3 +/- 0.2 microns/day) were all severely decreased in diabetic rats. Plasma osteocalcin concentrations were also markedly decreased in diabetic rats (24 +/- 2 vs. 108 +/- 10 ng/ml); the half-times of [125I]osteocalcin were similar in diabetic and nondiabetic rats, indicating that decreased plasma osteocalcin was due to decreased synthesis. Plasma osteocalcin levels were more decreased than expected from their suppressed 1,25-dihydroxyvitamin D3 levels, and 1,25-dihydroxyvitamin D3 injections did not increase plasma osteocalcin in diabetic rats as they did in nondiabetic rats. Bone osteocalcin content was normal in diabetic rats. Photon absorptiometry of tibiae showed a similar bone mineral content in diabetic and nondiabetic rats. Biomechanical properties of diabetic rat femora were all in the normal range. Nondiabetic semistarved rats with the same body weight as diabetic rats exhibited a similar delay in bone growth as diabetic rats, but the osteoblast and osteoid surfaces were normal, and the mineral apposition rate was normal (tibia) or slightly decreased (vertebra). Plasma osteocalcin concentrations were also normal in semistarved rats. Thus, the number and/or function of osteoblasts are severely suppressed in diabetes, and this results in decreased osteoid surface, mineral apposition rate, and plasma osteocalcin levels; moreover, these changes cannot be explained by simple weight loss.