This paper describes a study on metabolism of inhaled tritiated hydrogen gas (HT) in mammals. Rats and human volunteers were exposed to HT gas for periods from one minute to ten minutes. The amount of HT that was oxidized in vivo was estimated by measuring the concentration of HTO in urine. No significant quantities of tritiated organic compounds resulting from HT gas inhalation were observed in tissues of rats and it is assumed that this holds true for humans also. The fraction of inhaled HT converted to HTO in human volunteers was found to be about 1 × 10−4 indicating that the dose from the HTO that resulted from HT oxidiation is a significant component of the effective dose equivalent.