Heme oxygenase is the rate-limiting enzyme in the heme degradative pathway. It catalyzes the degradation of heme into two important products, biliverdin and C0.'. 2 By the action of biliverdin reductase, biliverdin is readily converted to bilirubin, a potent antio~ idant.~~ The second product, CO, has been recently reported to be a putative neurotransmitter. 6 Heme oxygenase consists of two isozymes, an inducible heme oxygenase-1 (HO-1) and a constitutive heme oxygenase-2 (HO-2). 7 HO-1 is induced under conditions of oxidative stress. 2 Recent studies have shown that the increase in mRNA for HO-1 is a response to the depletion of cellular glutathione levels resulting from oxidative HO-2 mRNA (two different sizes) remains relatively unchanged under these condition^.^ Thus, it would appear that the increase in the ratio of concentrations of HO-1 mRNA to that of HO-2 mRNA could be used as a useful index of oxidative stress. Therefore, we have developed a reverse transcription-polymerase chain reaction (RT-PCR) technique to estimate this ratio and its response to oxidative damage. Cultured human retinal pigment epithelial (RPE) cells were employed since they are a well-characterized neuroepithelial cell type with inducible HO activity.