Recent developments in the technique of constant infusion and in emission computed tomography have provided a new method for quantitative regional flow measurements. This paper re-examines critically the theoretical relationship between flow and radioactivity during constant infusion of short-lived isotopes. Sensitivity analysis is used to assess the accuracy of the method. Criteria for the selection of an appropriate half-life to meet accuracy requirements are also discussed. A method for the determination of the distribution volume of the tracer is proposed.