Ferrous Hb contributes to cerebral vasospasm after subarachnoid hemorrhage, although the mechanisms involved are uncertain. The hypothesis that cytotoxic effects of ferrous Hb on smooth muscle cells contribute to vasospasm was assessed. Cultured rat basilar artery smooth muscle cells were exposed to pure Hb, dog erythrocyte hemolysate, or Hb breakdown products; and heme oxygenase (HO-1 and HO-2) and ferritin mRNA and protein were measured. Cytotoxicity was assessed by lactate dehydrogenase release and fluorescence assays. Pure Hb or hemolysate caused dose- and time-dependent increases in HO-1 mRNA and protein. Hemin was the component of Hb that increased HO-1 mRNA. Cycloheximide inhibited the increase in HO-1 mRNA in response to hemin. Ferritin protein heavy chain but not mRNA increased upon exposure of cells to Hb. Hemin and ferric but not ferrous Hb were toxic to smooth muscle cells. Toxicity was increased by exposure to Hb plus tin protoporphyrin IX. In conclusion, exposure of smooth muscle cells to Hb induces HO-1 mRNA and protein through pathways that involve new protein synthesis. Hemin is the component of Hb that induces HO-1. Hemin and ferric but not ferrous Hb are toxic.