Recently, heme oxygenase-1 (HO-1) has been shown to be present in vascular smooth muscle cells. In the present study, we examined the effect of angiotensin II (Ang II) on HO-1 in rat vascular smooth muscle cells. After treatment with 100 nmol/L Ang II, HO-1 mRNA levels were decreased, with a nadir at 2 hours (39±9% of the control level, P <.01). This downregulation was completely blocked by the Ang II type 1 receptor antagonist losartan. Western blot analysis showed that HO-1 protein is also significantly downregulated, with a nadir at 4 hours (52±6% of the control level, P <.01). Heme oxygenase activity was also significantly decreased at 4 hours (control, 0.35±0.86 nmol bilirubin/mg per hour; Ang II, 0.10±0.06). This downregulation was observed in serum-starved cells to a similar extent as in serum-supplemented cells. Inhibitors of protein kinase C, lipoxygenase, cyclooxygenase, cytochrome P450 monooxygenase, and phospholipase A ₂ did not block this downregulation. However, this effect was not observed in the absence of calcium and presence of EGTA (2 mmol/L). Furthermore, a 2-hour incubation with calcium ionophore or arginine vasopressin decreased HO-1 mRNA levels, suggesting that an increase of intracellular calcium mediates the downregulation. In conclusion, Ang II decreases HO-1 mRNA in a calcium-dependent manner in vascular smooth muscle cells, which may provide a novel mechanism for the modulation of vascular tone and oxidative stress.