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The in vitro effect of Praziquantel (PZQ) onTaenia solium cysticerci was analyzed. The oxygen consumption rate of the parasites was inhibited and the release of proteins was enhanced, but no statistically significant differences were found between the control group and the experimental groups. The drug had a significant, dose-dependent negative effect on the evagination ability of the larvae; 50% effect was seen at concentrations of between 10-9 and 10-8M PZQ. The drug also induced morphological disturbances in the tegument of the worm and of the bladder wall. Finally, a very drastic effect was the induction of spastic paralysis in evaginated cysticerci at high drug doses and of flaccid paralysis at lower PZQ concentrations. The dose inducing these effects was various orders of magnitude lower than that inhibiting the evagination ability of intact cysts. Moreover, the latter effect was reversible after had been washed out the drug and the parasites had been cultured. We suggest that PZQ alters the Ca2+ flux inT. solium as it does in other helminths. Furthermore, we corroborated the protective role of the bladder for the invaginated worm. Finally, we think that in vivo the drug must act synergistically with the immune system components so as to eliminate the parasite.