The toxicity and anthelminthic activity of the earlier synthetized tricyclic analogues of praziquantel and 4-acylpiperazinones-2 have been studied. Tricyclic compounds have shown the acute toxicity similar to that of praziquantel and neurotoxic effect typical of praziquantel. 4-acylpiperazinones-2 toxicity correlated with their anthelminthic effect. The determination of anthelminthic activity of the above compounds in opisthorchiasis and hymenolepiasis has shown that they are less effective than praziquantel or have no anthelminthic activity. A biological activity-structure relationship has been traced in the compounds under study.