Abstract: It was found by in vitro and in vivo tests, that pyquiton neither increased the endogenous 5-hydroxytryptamine (5-HT) concentration of the worms nor did it increase the uptake of exogenous 5-HT. It was suggested, therefore, that 5-HT might not play an important role in the stimulation and spasmodic contraction of the worms induced by pyquiton.The 5-HT concentration of male and female Schistosoma japonicum was 2.28±0.23 and 1.08±0.11 μg/g (wet weight) respectively, while that of the paired worms, 1.59±0.44 μg/g (wet weight). The endogenous 5-HT level of schistosomes was rather stable and no apparent change was observed after the worms were maintained in culture medium for 6 hours. When schistosomes were incubated in the medium containing a high concentration of 5-HT (5×10^-4 M or 1×10^-3 M) for 4 hours, the amount of 5-HT removed by the worms was significant. However, the 5-HT take up by the worms was released promptly after transferred to a 5-HT-free medium.