A series of experiments on exposure of man and rats to tritium gas (HT) or tritiated water (HTO) are reported. In one human experiment 3 millicuries of HTO was administered in 200 milliliters of water by ingestion. Absorption into the blood stream was linear with time and complete in about 45 minutes. Body water turnover was measured over a period of six days by weighing all food and fluid intake and all excretory output. For four days water turnover was kept near normal (2.7 liters per day). During the last two days water turnover was increased to 12.8 liters per day. Tritium excretion rates was determined on eight other human subjects in which water turnover in which measured less precisely. The biological half-life of HTO in nine human subjects varied from 9 to 14 days on ad libitum and was reduced to 2 1/2 days in one subject on high water intake. The tritium activity in sweat, expired water vapor, septum and urine was found to be essentially the same as that in water from the blood. Rats were continually exposed to various concentrations of tritium in inspired air (0.000001 to 0.03 microcuries per ml) for periods up to 145more » hours. Inhaled tritium appeared in the animal as HTO apparently being converted thus in the body.« less