Abstract A new and realistic model of interstitial albumin distribution is developed in which the interstitial albumin is pictured as flowing in innumerable discrete streams through the interstitial fluids. The differential difference and differential integral equations describing this model are derived, and analytic and numeric solutions are obtained for a number of cases. These solutions are used to determine the flux of albumin and the distribution of passage times of the albumin streams through the interstitial fluids, as well as the catabolic rate of albumin, the total interstitial albumin, and, approximately, the radioactivity of the returning lymph albumin. The model satisfactorily meets all the tests that can be applied at present.