Interstitial fluid oncotic pressures in rabbit subcutaneous tissues were measured by chronically implanted membrane osmometers. Pairs of osmometers were used, one using a membrane permeable and one impermeable to plasma proteins. Measurements from the small-pore membranes averaged 10.2 mmHg while pressures measured from the large-pore membrane osmometers averaged -1.2 mmHg, indicating that the measured pressures were osmotic rather than hydrostatic in nature. These values are in agreement with previously published values from this laboratory on oncotic pressures of excised rabbit skin and with a computer-simulation study of capillary fluid balance. The oncotic pressures described in this study are of the s-me general magnitude as those of lymph from most organs in the body.