The water balance in Wistar, Zucker obese and Zucker lean rats, aged 60 days, was measured by determining the amount of water they drank, that contained in the solid food eaten, the water lost through urine and droppings, the net water accrued (estimated from the composition of the body and the daily increase in body weight), the measurement of the water vapour lost and the calculation of metabolic water production by means of the measurement of oxygen consumption, carbon dioxide production and protein oxidation in a 24 hr period. 1. Despite widely different body weights, all three groups of animals accrued a similar proportion of their daily water budget (4.3-4.9%, i.e. 1.2-1.4% of the total rat water mass). 2. Wistar and Zucker obese rats had a similar daily water budget despite very different body weights, lean Zucker rats had lower water budgets. 3. Obese and lean Zucker rats produced a more concentrated, and excreted much less urine (the highest urea concentration was found in obese rats) than Wistar rats. 4. The water lost in the droppings was in the same range as that in urine for obese rats, slightly less for lean Zucker rats and much less in Wistar rats. Obese rats produced a higher amount of stool with respect to the amount of food eaten than the lean animals studied. 5. The contribution of metabolic water to the daily water budget was a 23.6% for Zucker obese, 22.5% for Zucker lean and 15.9% for Wistar rats.