Sheep were prepared with a chronic lung lymph fistula and studied unanesthetized following septicemia by infusion of live Escherichia coli 10(9) ml/kg bw or injection of oleic acid 0.05 ml/kg bw. Extravascular lung water (EVTV) was measured with thermal-dye technique and compared to gravimetrically measured lung water (EVWV). Septic sheep had increased pulmonary artery pressure, reduced mean arterial blood pressure and reduced cardiac output. In control animals there was a correlation between EVTV-EVWV of r = 0.70. In animals given oleic acid lungs were macroscopically edematous and the correlation was r = 0.93. In septic sheep, however, no correlation could be found between EVTV and EVWV (r = -0.25). The thermal-dye technique was found to give erroneously high values. This finding could probably be due to erythrostasis and leukocyte plugging with uneven perfusion and prolonged transit times due to reduced cardiac output.