A mean transcapillary exchange rate for albumin and gamma globulin has been obtained from plasma disappearance curves of I131-labeled albumin and I125-labeled gamma globulin. A much closer correlation exists between transcapillary exchange rate for albumin and the plasma volume than between the rate and serum albumin concentrations. Also, for albumin this rate varies with plasma volume and circulating protein in such a manner as to suggest that the capillary permeability changes with changing plasma volume. The relation between vascular and extravascular albumin is consistent with this interpretation, the extravascular mass of albumin appearing to approach zero as vascular albumin approaches the vicinity of 0.35 Gm/Kg. In experiments in which I131-labeled albumin and I125-labeled globulin were employed to obtain simultaneous data on albumin and gamma globulin, a good correlation has been obtained between the simultaneously measured albumin and globulin transcapillary exchange rates. The ratio of the fractional capillary exchange rates of these 2 substances is very nearly the same as the ratio of their diffusion coefficients, a relation to be expected if diffusion is the mechanism of transcapillary exchange.