People working near heavy water reactors may be internally contaminated by tritium absorbed through the lungs and skin following accidental release of tritiated water. To establish the possible extent of this contamination volunteers have been exposed to tritium in air concentrations 0.5–50 μc/l. for 1.50 min. Tritium uptakes were determined by measuring the concentration of tritium in urine and estimating the volume of body water. Tritium was also measured in exhaled water vapour. The intake rates for three unprotected subjects were 14.8, 14.8 and 17.9 μc/min per μc/l. The average skin intake rate for seventeen subjects was 9.6 (in the same units), individual rates being within the range 6.6–13.4. In exposures when clothing and oronasal masks were worn the intakes were as follows: cotton overalls-5.4–13.0; 4-mil polyvinyl chloride (PVC) suits-0.6, 1.0; 4-mil PVC suit over cotton overalls-0.09–0.19; 6-mil, two-piece PVC suit over cotton overalls-0.2, 0.5; overalls between two 4-mil PVC suits-0.01–0.04. Tritium dispersed in the body fluids with half periods between 19 and 55 min and was eliminated from the body with half lives between 6.4 and 14.4 days.