P-glycoprotein, the human multidrug resistance (MDR1) gene product, is an integral membrane protein expressed on the plasma membrane of MDR tumor cells and is the best characterized of a family of efflux transporters that confer chemotherapeutic resistance. The use of gamma-emitting 99mTc-agents to image P-glycoprotein function in human tumors in vivo has been proposed. Net tumor cell content of 99mTc-Sestamibi, 99mTc-Tetrofosmin and several 99mTc-Q-complexes 99mTc-Q58 and 99mTc-Q63) are a function of passive potential-dependent influx and MDR1 P-glycoprotein-mediated active extrusion. To better understand the overall fidelity of these P-glycoprotein substrates to report MDR activity in vivo in relation to tissue perfusion, a compartmental model of tracer pharmacokinetics was developed. Modeling indicates that tissue perfusion will impact pharmacokinetics in vivo in a manner that will tend to diminish P-glycoprotein-mediated phenotypic differences between tissues when they are perfusion-limited. However, dynamic imaging to extract efflux rate constants is independent of perfusion and may represent the highest quality methodology for collecting the desired information regarding activity of the efflux transposter. Much work remains to translate these concepts and biological targeting properties into clinical practice.