1. Deuterium oxide was used to estimate body water in twenty-four pigs of widely differing body composition and of average weight 83·9 kg.
2. After infusion of the isotope, blood samples were collected every 30 min for 4 h. The resulting plasma was purified by a heat-distillation procedure, after which it was analysed for D2O by infrared spectroscopy.
3. Approximately 24 h after infusion of the D2O each pig was killed, and its composition determined both by chemical analysis and physical dissection.
4. Equilibration of D2O in the body was found to be complete within 2 h of injection of the tracer.
5. The mean D2O space was found to be 8·6% greater than the mean empty body water space, but only 2·2% greater than the total body water space.
6. Empty body water and total body water were estimated from the regression lines with residual standard deviations of 2·7 and 1·9% respectively. Similarly, the residual standard deviations of the regressions involving the other fat-free components were 6·3% for dissectible lean, 3·2% for fat-free mass, and 5·6% for crude protein.
7. The residual standard deviations of the regressions in which the weights of dissectible fat and total body lipid were predicted were 6·0 and 6·7% respectively.