One of the most interesting facts about the renal circulation is that during marked changes in renal blood flow (adrenalin ischemia and pyrogenic hyperemia) the rate of glomerular filtration typically remains unchanged. This fact has been attributed to the circumstance that the changes in renal blood flow are mediated primarily by changes in the tonus of the efferent arterioles; consequently, any increase or decrease in blood flow is accompanied by a reciprocal change in glomerular filtration pressure, with the result that the filtration rate remains unchanged (3).Beyond the fact that this emphasis upon the efferent arteriole is to some extent contrary to the importance which has hitherto, chiefly for anatomical reasons, been attached to the afferent vessel, this description of the glomerular circulation presents several interesting implications. It assumes that the rate of glomerular filtration is determined solely by glomerular pressure factors, exclusive of any limitation imposed by the permeability of the glomerular membranes. There logically issues from this assumption the question whether or not filtration pressure equilibrium is normally reached in the glomerulus. The answer to this question is of practical importance in two respects. If filtration equilibrium is reached in the glomerular circulation, then this fact must set the upper limit to the hydrostatic pressure available to propel blood through the efferent arterioles and the postglomerular circulation. And in the face of a demonstration of filtration equilibrium in the normal kidney, a decrease in filtration rate in renal disease cannot logically be attributed to reduced permeability of the glomerular capillaries, in contradistinction to a decrease in filtration pressure or in total filtering 1This investigation has been supported in part by aid from the William Gibbs Memorial Prize Fund, granted especially for the study of renal hyperemia, and in part by the Commonwealth Fund.surface, without evidence that glomerular permeability is actually reduced.These and other considerations lead us to a further analysis of this problem. It has recently been shown that the inulin and diodrast clearances in different subjects are closely correlated with differences in the tubular excretory mass (diodrast Tm), it being presumed that the last term varies with the total quantity of renal parenchyma (8). It is advantageous to utilize this correlation in examining the relations which exist in various subjects between the inulin and diodrast clearances; i.e., to make this comparison in terms of the ratios, CINITmD and CDITmD (where CIN and CD are the inulin and diodrast clearances, respectively, and TmD is diodrast Tm). Such an analysis is, in the first approximation, equivalent to comparing the filtration rate and renal plasma flow of various subjects on the basis of their actual renal weights.Furthermore, we now have in our records numerous data, not available when the previous paper was published, on the diodrast and inulin clearances in normal subjects under va...