The antifilarial compound diethylcarbamazine (N,N-diethyl-4-methyl-2-piperazine, DEC) is known rather for its micro- than macrofilaricidal activity. But in some human filariasis i.e. loaiasis, lymphatic filariasis, the spectrum of DEC activities extends to adult filaria. The potential role of the metabolites of DEC in the action of the parent drug once it had been metabolized in the body of infected animals was investigated. The metabolites were evaluated in a new experimental model on which DEC is active: Molinema dessetae in its natural host, the rodent Proechimys oris. Experimental studies were carried out in vivo and in vitro, on microfilariae, infective larvae and adult filaria. Several other nematodes were also used. The metabolites were DEC itself, N-ethyl-4-methyl-1-piperazine-carboxamide (MEC) and their N-oxides, 4-methyl-piperazine-carboxamide and N,N-diethyl-1-piperazine-carboxamide. In vivo most of the metabolites were found active on microfilariae and both N-oxides active on adults and infective larvae. In vitro, the activity of the metabolites was observed only with high concentrations; the in vitro test could not be used as a screening method for antifilarial chemotherapy with piperazine derivatives. Infective larvae were the most sensitive stage. In the rodent and in man, the antifilarial action of DEC is swift and of short duration. This action is prolonged by the activity of metabolites, especially the N-oxides.