The purposes of this study were to clarify that warfarin (WF, vitamin K antagonist) levels that inhibit gamma-carboxylation are different in liver and bone (experiment 1), and to investigate whether the plasma osteocalcin (OC) level reflects bone OC levels (experiments 2 and 3). Four-week-old male rats were treated with 0.2, 0.4, 0.6, 0.8, 1.0, or 1.2 mg/l of WF solution as drinking water for 4 weeks. Blood coagulation activity, an index of gamma-carboxylation of prothrombin in the liver, was significantly decreased in rats receiving 0.8 mg/l or larger doses of WF. A significant decrease of plasma gamma-carboxylated OC (GlaOC), an index of gamma-carboxylation of OC in bone, was shown in rats receiving 0.2 mg/l or larger doses. Significantly lower OC levels in the femoral diaphysis and metaphysis were shown in the 0.2 mg/l and 0.4 mg/l groups. However, femoral bone mineral density (BMD) values in the WF-treated groups were almost the same as those in the intact group. In experiment 2, we evaluated changes in bone OC levels 4 weeks after discontinuing an 8-week WF treatment. Four-week-old male rats received 0.8 mg/l WF as drinking water for 8 or 12 weeks. Recovery of the OC level after discontinuing the WF treatment was shown in the femoral metaphysis, but not in the diaphysis. In experiment 3, 0.3 mg/kg WF was administrated to 25-week-old male rats three times a week for 8, 12, or 16 weeks. In aged rats, decreased bone OC was shown in the femoral metaphysis, but not in the diaphysis. From these findings, it is suggested that the effects of WF on gamma-carboxylation are likely to appear in bone at lower doses than in the liver, that the bone OC level does not always correspond directly to plasma GlaOC, and that the bone OC level is not directly linked with BMD.