This report presents various laboratory measurements taken during the first 48 hours after thermal injury on a series of 153 adults and 185 children from Peru with burns involving 10 per cent or more of the body surface area. The effect of fluid therapy upon changes in the circulation and fluid and electrolyte balance was assessed by simultaneous comparison of groups of patients to whom different types of fluids were administered. The fluid regimens employed consisted essentially of one type containing large amounts of sodium with or without added protein and another type with only small quantities of sodium but with varying quantities of protein. Measurements made soon after thermal trauma but before fluid therapy had been started demonstrated (1) a decreased plasma volume in adults which was proportional to the area burned only in patients with up to 50 per cent of the body surface area involved, (2) hemoconcentration in adults and children proportional to the area burned, and (3) plasma levels of proteins, sodium, potassium, and chloride in adults and children more or less within the normal range with no changes correlated with the extent of the burn. Only plasma bicarbonate was decreased with a trend to lower values in patients with the more severe burns. Fluid therapy with large quantities of sodium in adults produced over-all fluid and sodium retention with better restoration of plasma volume to normal during the shock period. Glomerular filtration was increased and renal plasma flow normal in patients who were not in shock at the time of the determination. Hemoconcentration in adults and children was quickly corrected, and plasma electrolytes, for the most part, were maintained within the normal range. Total plasma proteins dropped markedly during the early postburn therapy period from the initial value. On the other hand, fluid therapy with plasma but with only small quantities of sodium in adults produced no over-all water or sodium retention. There was less efficient restoration of plasma volume. The patients who died were included in this therapy group, and their plasma volumes at various times during the shock period were always markedly diminished. Glomerular filtration was normal, but renal plasma flow was decreased in the few patients not in shock. Hemoconcentration in adults and children was more persistent, and a high incidence of hyponatremia and hypochloremia was noted. Total plasma proteins remained fairly close to the normal range. From this data it is possible to conclude that the composition of the fluid therapy administered during the shock period does indeed influence the response of the circulation and fluid and electrolyte balance. In adults, therapy with large quantities of sodium was more effective in preventing death during the shock period than therapy with plasma supplemented by glucose and water. The less favorable response in the plasma group was most likely due to the low sodium content of the fluid administered to that group. The saline-treated patients also showed better restoration of the circulation and fluid and electrolyte balance. Statistical analysis of the differences in hematocrit, plasma proteins, and plasma sodium of the patients who lived or died, however, did not demonstrate any significant difference by which survival or death could be predicted. © 1961.