Both singlet oxygen and the hydroxyl radical are generated in mammalian cells by UVA (320-380 nm) and possibly near-visible (380-420 nm) radiation. We have modulated the cellular levels of these two reactive oxygen species in order to compare their involvement in the induction of the human heme oxygenase (HO) gene by broad spectrum UVA/near-visible light (UVA/NVL). Irradiation in deuterium oxide (in which singlet oxygen has a longer half-life) enhances the broad spectrum UVA/NVL induction of this gene. Sodium azide and L-histidine which are scavengers of both singlet oxygen and the hydroxyl radical reduce the fluence-dependent accumulation of HO mRNA, while compounds which are only hydroxyl radical scavengers, namely, mannitol and dimethyl sulfoxide do not. Rose Bengal, a known generator of singlet oxygen, also increases the HO mRNA levels, and this induction is enhanced in deuterium oxide. We conclude that the observed effects of deuterium oxide and singlet oxygen scavengers on HO mRNA levels are not due to a nonspecific effect on transcription but that singlet oxygen is a primary effector in the UVA/NVL induction of the human heme oxygenase gene.