Studies of the effects of the administration intravenously of isotonic and slightly hypertonic solutions of crystalloids on the blood volume and on the dynamics of the circulation in normal man have been reported recently from this laboratory.1 After such injections there were noted increased blood volume, with decreased hematocrit reading and plasma protein concentration, increased cardiac output and velocity of blood flow, widening of pulse pressure and transient increase in venous pressure. In the present investigation the effect of the administration intravenously of isotonic solutions of sodium chloride on renal function in normal man has been studied. The changes which occur in the systemic circulation after this procedure, if shared directly by the glomerular capillary circulation, should affect the glomerular filtration rate, according to the filtration-reabsorption theory of renal function. According to this theory, the glomerular filtration rate is determined by the total functioning glomerular capillary surface, the effective