Our aim was to establish in spontaneously-breathing urethane-anesthetized rats, the relationship between the concentrations of H₂S transported in the blood and the corresponding clinical manifestations, i.e. breathing stimulation followed by terminal apnea, during and following infusion of NaHS at increasing rates. The gaseous concentration of H₂S (CgH₂S, 1/3 of the total soluble form) was computed from the continuous determination of H₂S partial pressure in the alveolar gas, while H₂S, both dissolved and combined to hemoglobin, was measured at specific time points by sulfide complexation with monobromobimane (CMBBH₂S). We found that, using a potent reducing agent in-vitro, H₂S added to the whole blood had very little interaction with the plasma proteins, as sulfide appeared to be primarily combined then oxidized by hemoglobin. In vivo, H₂S was undetectable in the blood in its soluble form in baseline conditions, while CMBBH₂S averaged 0.7±0.5 µM. During NaHS infusion, H2S was primarily present in non-soluble form in the arterial blood; e.g. CMBBH₂S was about 50 times higher than CgH₂S at the lowest levels of exposure and 5-6 times at the levels wherein fatal apnea occurred. CgH₂S averaged only 1.1±0.7 µM when breathing increased, corresponding to a CMBBH2S of 11.1±5.4 µM. Apnea occurred at CgH₂S above 5.1 µM and CMBBH₂S aove 25.4 µM. At the cessation of exposure, CMBBH₂S remained elevated, at about 3 times above baseline for at least 15 min. These data provide a frame of reference for studying the putative effects of endogenous H₂S and for testing antidotes against its deadly effects.