Malaria is one of the most devastating tropical diseases despite the availability of numerous drugs acting against the protozoan parasite Plasmodium in its human host. However, the development of drug resistance renders most of the existing drugs useless. In the malaria parasite the tripeptide glutathione is not only involved in maintaining an adequate intracellular redox environment and protecting the cell against oxidative stress, but it has also been shown that it degrades non-polymerized ferriprotoporphyrin IX (FP IX) and is thus implicated in the development of chloroquine resistance. Glutathione levels in Plasmodium -infected red blood cells are regulated by glutathione synthesis, glutathione reduction and glutathione efflux. Therefore the effects of drugs that interfere with these metabolic processes were studied to establish possible differences in the regulation of the glutathione metabolism of a chloroquine-sensitive and a chloroquine-resistant strain of Plasmodium falciparum. Growth inhibition of P. falciparum 3D7 by D,L-buthionine-( S, R )sulphoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase (gamma-GCS), and by Methylene Blue (MB), an inhibitor of gluta thione reductase (GR), was significantly more pronounced than inhibition of P. falciparum Dd2 growth by these drugs. These results correlate with the higher levels of total glutathione in P. falciparum Dd2. Short-term incubations of Percoll-enriched trophozoite-infected red blood cells in the presence of BSO, MB and N, N (1)-bis(2-chloroethyl)- N -nitrosourea and subsequent determinations of gamma-GCS activities, GR activities and glutathione disulphide efflux revealed that maintenance of intracellular glutathione in P. falciparum Dd2 is mainly dependent on glutathione synthesis whereas in P. falciparum 3D7 it is regulated via GR. Generally, P. falciparum Dd2 appears to be able to sustain its intracellular glutathione more efficiently than P. falciparum 3D7. In agreement with these findings is the differential susceptibility to oxidative stress of both parasite strains elicited by the glucose/glucose oxidase system.