Abstract 1. 1. Certain fluid kinetics have been studied in ten patients whose burns exceeded 15 per cent of body-surface areas. 2. 2. The observations and measurements performed included the clinical course, fluid intake, urinary output, body weights, urinary sodium excretion, right heart and pulmonary arterial pressures, and measurements of cardiac output by means of the ballistocardiograph, dye, and direct Fick methods. 3. 3. The three methods for the clinical measurement of cardiac output are compared. Early results indicate a fairly good correlation between the values obtained in normal subjects by the three methods. Cardiac output values obtained in burn patients have been above the lower limits of normal. 4. 4. The dye method for measuring cardiac output is particularly suited to clinical use. 5. 5. Evidence is presented which indicates that burn patients may be overtreated with fluids. This further substantiates the well-recognized fact that after the first forty-eight to seventy-two hours the amount of fluid and electrolyte administered to burn patients should be sharply reduced. 6. 6. In the most severely burned patients that we have studied, there has been a tendency for the diuresis to begin on the fifth to the seventh day and to be completed on the thirteenth to the sixteenth day. There was a concomitant sodium and water diuresis in the one patient placed on a partial sodium balance study. 7. 7. The rate of insensible fluid loss varies considerably from one burn patient to another and in the same patient from day to day. 8. 8. Pulmonary and right-heart pressures were not abnormal in three burn patients so studied. 9. 9. It is perhaps worth re-emphasizing that the systolic blood pressure, used in conjunction with the urinary output, is still a most useful guide to fluid therapy in severely burned patients.