131I labeled iodo-antipyrine and 99mTc labeled erythrocytes were used to measure water content in lungs. These radioactive tracers were injected into 10 rabbits with normal lungs and 11 rabbits with injured lungs. Blood samples were drawn and the subjects were killed. The lungs were removed, weighed and homogenized. Samples of blood and lung homogenate were assayed for 131I and 99mTc. Samples were also weighed before and after drying to a constant weight at 70-75 degrees C. Extravascular lung water was determined by the dual isotope technique and again by gravimetric analysis. The average ratio of the results from the 2 different methods is 1.03 +/- 0.15. The 2 methods were compared by regression analysis and the correlation coefficient was 0.92 +/- 0.09. Our investigation suggests the possibility of measurement of lung water with equilibrium distribution of iodo-antipyrine.