Two lactating Friesian cows received tritiated water (19 nCi of tritium per ml) to drink for 25 days. Urine and milk samples were collected and analyzed for tritium content. Plateau levels in milk water, and in milk fat, lactose and casein were reached in about 18 days. Tritium concentration in milk water and urine water was about 82% that in drinking water, indicating dilution with water from food and metabolic processes, and as a result of exchange through lungs and skin. At steady state, tritium activity in milk fat, lactose and casein was about 30, 28 and 15% of that in drinking water on a weight basis; at this time, about 1.5% of the daily ingested tritium appeared in one litre of milk in both animals. Comparision of the specific activity of the various milk constituents with the specific activity of the body water showed that the highest tritium incorporation occurred in lactose (0.58), followed by milk fat (0.36) and casein (0.27). Tritium incorporation in milk dry matter (0.45) is considerably higher than in most tissue components of several mammalian species after continuous ingestion of THO as reported in the literature. Tritium levels fell rapidly in milk and urine after the administration of THO was stopped, with half-times of around 4 days for the rapid components in all milk constituents. A slow component of 44 days, 225 days and 24 days half-time was found in milk water, milk fat and casein respectively.