Altered calcium [Ca²⁺] transients of vascular smooth muscle cells to vasoconstrictors may contribute to altered regulation of blood flow in diabetes. We postulated that diabetes-induced transforming growth factor (TGF)-β production contributes to impaired ANG II response of vascular smooth muscle cells in macrovessels and microvessels. Aortic vascular smooth muscle cells isolated from diabetic rats exhibited markedly impaired ANG II-induced cytosolic calcium [Ca²⁺] signal that was completely restored by pretreatment with anti-TGF-β antibodies. Similar findings were noted in microvascular smooth muscle cells isolated from preglomerular vessels and cultured in high glucose. The impact of diabetes on [Ca²⁺] transients was replicated by addition of TGF-β₁ and -β₂ isoforms to aortic smooth muscle cells in culture and diabetic cells had enhanced production of TGF-β₂. In the in vivo condition, TGF-β₁ was increased in diabetic glomeruli, whereas TGF-β₂ was increased in diabetic aorta. The characteristic increase in glomerular filtration surface area found in diabetic rats was prevented by treatment with anti-TGF-β antibodies, and impaired ANG II-induced aortic ring contraction in diabetic rats was completely restored by anti-TGF-β antibodies. Impaired vascular dysfunction may be partly due to decreased inositol 1,4,5-trisphosphate receptor (IP₃R), as reduced type I IP₃R expression was found in diabetic aorta and restored by anti-TGF-β antibodies. We conclude that TGF-β plays an important role in the vascular dysfunction of early diabetes by inhibiting calcium transients in vascular smooth muscle cells.