The aim of t his study was to investigate the cytotoxic and ni t ric oxide (NO)-inducing effects of bismuth oxide (Bi203)-containi ng Portland cement (BPC) on human dental pulp cells. We also assessed whether heme oxygenase-l (HO-l) is involved in BPC-induced cytotox.icity in dental pulp cells Cytotoxicity and NO production induced by BPC were higher than those induced by Portland cement (PC) at 12 and 24 hours, and the former grad ua lly decreased to the level observed for PC. HO- l and inducible nitric oxide synthase (iNOS) mRNA expressions in the BPC group showed maximal increase at 24 hours. and it gradually decreased with increasing cultivation tlme Hemin treatment reversed the BPC-induced cytotoxicity ‘ whereas zinc protoporphyrin IX treatment increased the cytotoxicity. These results suggested that NO production by BPC correlates with HO-l exp1'ession in dental pulp cells Moreover ‘ BPC- induced HO-l expression in dental pulp cells plays a protective 1'ole against the cytotox.ic effects of BPC.