Abstract: When male Schistosoma japonicum maintained in HBS at 4 or 37℃ and HBS without calcium were exposed to praziquantel 1 or 30 μg/ml for 0.5～2h, no apparent change in Ca²⁺ content of the worms was observed. With the exception of the group HBS at 4℃, a significant decrease in K⁺ content of the worms was detected in another 2 groups. On the other hand, Na⁺ content of the worms exhibited somewhat increase after treatment with praziquantel but not significantly. Incubation of male schistosomes in HBS with 30 mM Mg²⁺ resulted in an increase in Mg²⁺ content of the worms caused by praziquantel. Similar changes, compared to HBS group at 37℃, in the 4 element values were detected in male worms perfu sed out from infected mice treated orally with praziquantel 100～300 mg/kg. When male schistosomes maintained in HBS at 37℃ were exposed to prazquantel 1 μg/ml for 5～60 min the percentage of radioactivity distributed in tegumental cytoplasma decreased significantly while that of the worm musculature increased significantly. When male worms were incubated in HBS at 4℃ or HBS without calcium, no apparent change in distribution of radioactivity caused by praziquantel was detected.