In an experimental model we have demonstrated that visualization of the target organ is the only dependable criterion for the scintigraphic evaluation of LeVeen shunt patency or nonpatency. The lungs are best suited as the target, because the presence or absence of pulmonary uptake can be easily seen without interference from radiolabeled peritoneal fluid. Proof of shunt patency should not rely upon direct visualization of shunt tubing, because very high or low flow rates are both associated with poor visualization. Maximum accumulation of activity in the shunt tubing occurs with intermediate flow rates (approximately 60 cc/hr).