Hydrogen sulfide is gaining acceptance as an endogenously produced modulator of tissue function. The present paradigm of H2S as a tissue messenger consists of H2S being released from the desulfhydration of L-cysteine at a rate sufficient to maintain whole tissue hydrogen sulfide concentrations of 30 µM - >100 µM, and these tissue concentrations serve a messenger function. Utilizing physiological concentrations of L-cysteine and aerobic conditions, we found that catabolism of hydrogen sulfide by mouse liver and brain homogenates exceeded the rate of enzymatic release of this compound such that measureable hydrogen sulfide release was less with tissue-containing versus tissue-free buffers. Analyses of the gas space over rapidly homogenized mouse brain and liver indicated that in situ tissue hydrogen sulfide concentrations were only about 15 nM. Human alveolar air measurements indicated negligible free H2S concentrations in blood. We conclude rapid tissue catabolism of hydrogen sulfide maintains whole tissue brain and liver concentrations that are three orders of magnitude less than conventionally accepted values and only 1/5000 of the hydrogen sulfide concentration (100 µM) required to alter cellular function, in vitro. For hydrogen sulfide to serve as an endogenously produced messenger, tissue production and catabolism must result in intracellular microenvironments with a sufficiently high hydrogen sulfide concentration to activate a local signaling mechanism while whole tissue concentrations remain very low.