Infants with hemolytic diseases frequently develop hyperbilirubinemia and are treated with phototherapy, which only eliminates bilirubin after its production. A better strategy might be to directly inhibit heme oxygenase (HO), the rate-limiting enzyme in bilirubin production. Metalloporphyrins (Mps) are heme analogs that competitively inhibit HO activity in vitro and in vivo and suppress plasma bilirubin levels in vivo. A promising Mp, zinc deuteroporphyrin bis glycol (ZnBG), is orally absorbed and effectively inhibits HO activity at relatively low doses. We determined the I(50) (the dose needed to inhibit HO activity by 50%) of orally administered ZnBG in vivo and then evaluated ZnBG's effects on in vivo bilirubin production, HO activity, HO protein levels, and HO-1 gene expression in newborn mice after heme loading, a model analogous to a hemolytic infant. The I(50) of ZnBG was found to be 4.0 μmol/kg body weight (BW). At a dose of 15 μmol/kg BW, ZnBG reduced in vivo bilirubin production, inhibited heme-induced liver HO activity and spleen HO activity to and below baseline, respectively, transiently induced liver and spleen HO-1 gene transcription, and induced liver and spleen HO-1 protein levels. We conclude that ZnBG may be an attractive compound for treating severe neonatal hyperbilirubinemia caused by hemolytic disease.