Determinations were made of the 2-hour 22Na space and the 1 and 2 hour 51Cr EDTA spaces in a number of tissues. In regions where the two 51Cr-EDTA spaces were similar and smaller than or similar to the 22Na space the former spaces were regarded as measures of the extra-cellular volume. Using previously published data for plasma volumes, extravascular plasma-equivalent albumin and IgG spaces and the turnover rate constants for the extravascular proteins, it was possible to calculate extravascular protein concentrations and rates of net filtration from the capillaries or lymph flow in some tissues. The albumin concentrations, probably underestimated, were about 40% of the plasma concentrations in small intestine, lung and skeletal muscle, higher in heart muscle and lower in stomach wall. The net filtration or lymph flow, probably overestimated, was 0.81, 1.5, 0.53, 0.61 and 0.12 microliters/min per g in heart muscle, small intestine, lung, stomach wall and skeletal muscle, respectively.