The inhibitor effect on the Glycogen synthetase (E.C.2.4.1.11) activity by four anthelmintics: Mebendazole, Parbendazole, Albendazole and Thiabendazole was determinated. The most inhibitor effect of Parbendazole on A. suum, F. hepatica and M. expansa, was observed. The lowest inhibitor effect of Thiabendazole, on all cases, was also observed. The aim of our study, and further studies on the same line, are to provide new reports about the unknown locus of actuation of these anthelmintics.