Several independent lines of evidence indicate the direct impairment by extracellular glucose at high concentrations of different osteoblastic functions with a marked decrease in bone mass toward osteoporosis, while the underlying mechanisms are not well clarified to date. We have previously demonstrated the functional expression of the neural amino acid γ-aminobutyric acid (GABA) signaling system including betaine/GABA transporter-1 (BGT-1) with a temperature-, sodium- and chloride-dependent activity of [³H]GABA accumulation in cultured rat calvarial osteoblasts. In this study, therefore, we attempted to demonstrate the possible involvement of BGT-1 isoform in bone dysfunctions due to impaired mineralization in rat calvarial osteoblasts cultured under hyperglycemic conditions. No significant change was seen in [³H]GABA accumulation in osteoblasts cultured for 7 d in vitro (DIV) under hyperglycemic conditions (glucose=25.5—50.5 mM) compared to those cultured in normoglycemic (glucose=5.5 mM) and hyperosmotic (mannitol=25.5—50.5 mM) conditions. In osteoblasts cultured for 14 DIV under hyperglycemic conditions, however, [³H]GABA accumulation was significantly increased compared to those cultured under normoglycemic and hyperosmotic conditions. Kinetic analysis revealed that hyperglycemic cultivation resulted in a significant increase in V_max values from 2.85 nmol/min/mg protein for normoglycemic conditions to 4.17 nmol/min/mg protein for hyperglycemic conditions without affecting K_m values. However, experimental hyperglycemia did not significantly affect the expression of mRNA for BGT-1 isoform by osteoblasts. These results suggest that GABA transport system may at least in part play a role in pathological malfunctions and abnormalities through a mechanism not directly related to gene expression in osteoblasts under hyperglycemia.