This study addresses the following question: Could the acute depression in breathing produced by hyperoxia, a reflection of the tonic drive to breathe from the arterial chemoreceptors, be accounted for by the presence a background level of local endogenous H2S? To address this question, we produced a stable but moderate increase in breathing (24 ± 11%) via continuous infusion of low levels of H2S, in 10 spontaneously breathing urethane-sedated rats. We found that acute exposure to 100% O2 (20 tests) decreased minute ventilation (VI) from 301 ± 51 to 210 ± 43 ml/min within 15 seconds in control conditions, but no additional significant drop in VI was observed during H2S induced hyperpnea. In addition, no decrease in the estimated concentrations of gaseous H2S in the arterial blood was observed during the hyperoxic tests. It is concluded that the ventilatory depression induced by high O2 appears to be limited to the tonic background peripheral chemosensory drive to breathe, but has little or no impact on the CB stimulation produced by low levels of H2S.