Authors carried out the experimental study to investigate whether the Paragonimus westermani would uptake the blood in the parasitic definitive host, and also tried for the amount and distribution of the blood which would be harbored in a worm body, and so on. Experimental animals used in this study were dogs and cats, and they were infected with metacercariae of P. westermani. Several months later of the infection, Cr(51) tagged red blood cells were administered to each animal, and the animals were sacrificed and autopsy was done after 24 or 48 hrs of the administration. One part of the detected P. westermani adult worms of each animal were bred in Tyrode's solution and the others were killed by fixing in formalin solution. By Well-type scintillation counter, the radioactivities were measured for the alive and killed worms and of the blood of each experimental animal which was collected at autopsy. In conclusion, it was found that P. westermani worms uptake blood in parasitic definitive host, and amount of the blood harboring in a worm of 56 infection-days(size of worm; 4 x 3 x 1 mm) was 0.002 ml, 0.004 or 0.005 ml in 103 infection-days(size of worm: 8 x 5 x 3 mm) and 0.011 ml in 217 infection-days(size of worm: 9 x 7 x 4 mm). Almost of the whole blood uptaked in the worms was distributed in intestine. It was observed that the blood uptaked in the worm body was excreted continuously through the excretory pore, so that the radioactivity could not measured almost completely in cases of 24 hrs-breeding worms.