A study was performed of the activities of both cytoplasmic and mitochondrial, malate dehydrogenase (MDH) (E.C.1.1.1.37) in purified extracts of whole specimens of male and female nematodes of four species: T. canis, T. cati, T. leonina and A. suum (and tissues), two trematodes: F. hepatica and D. dendriticum, and four cestodes: M. expansa, M. benedeni, D. caninum and T. hydatigena. The results show that there exist species and sexual differences in the enzyme activities of both enzymes. The relative importance of this energy pathways of these helminth species is discussed. Determinations were made of the in vitro inhibitory activities of four benzimidazoles and six synthesised pyrimidine derivatives on MDH (soluble and mitochondrial) from helminth parasites. Several pyrimidine derivatives (6-amino-5-methyl-5-nitro-uracil, 4-amino-1-methyl-2-methylthio-5-nitro-6-oxo-1,2,3,4-tetrahydropyrimid ine and 4-amino-2-methylthio-5-nitro-6-oxo-1,2,3,4-tetrahidropyrimidine) produced double percent in vitro inhibitions of those shown by the benzimidazoles.