Abstract Conceptual and mathematical techniques for processing data on diffusible radiotracer uptake, obtained from perfusion studies, were outlined. From the Renkin equation, a theoretical relationship between tissue clearance of molecules and the rate of flow, derivative equations were obtained which expedited calculations. One easily plotted approximation gave tissue clearance as a function of the reciprocal of flow. Deviations from “ideality” were discussed as being due to 3 contributing factors: dependence of the permeability-surface area coefficient on flow, continued extraction at near zero flow, and the nonideal behavior of membranes (not fully permeable, and the presence of back diffusion). Tissue extraction of both 42K and 86Rb was characterized by the clearance value at extrapolated zero flow, by the extraction fraction at zero flow, and the slope of the line of extraction versus flow. Although the situation is likely more complex with ammonia (since it can exist in both ionized and nonionized forms), this treatment gave a reasonable description of 13N-ammonia uptake by normal canine myocardium.