The time that blood elements remain in an artificial ventricle determines the period during which these blood elements are exposed to damaging factors. We present, for the first time, preliminary studies on the residence time distribution (RTD) of a ventricle as a function of type and orientation of valves (Bjork-Shiley and polyurethane valves). We constructed an open loop mock circulation system which simulates the flow and pressure through the left ventricle, and a drive triggered injection unit which can inject a chemical tracer or radioisotope during one diastolic filling phase. RTD studies were done using a Tc-99m radionuclide tracer and a chemical tracer. A scintillation camera clearly showed the influence of backflow through the artificial heart valves, and the intraventricular washout. Results indicate that 42% of the fluid elements are expelled at the end of the first ventricular beat, i.e., 58% of blood elements stayed behind to experience the second beat; 36% of blood elements expelled in the second beat belonged to those in the first diastole; 17% in the third beat; and 5% in a subsequent number of beats. The RTD was found to vary with the position of the Bjork-Shiley (BS) valves.