Substances like imidazoles, benzimidazoles and also quinolines, whose chemical structure includes a heterocyclic nitrogen, are known to interfere with the microsomal oxidation and, in some cases, with the metabolism of drugs. Since chloroquine and primaquine exert this effect in vivo and in vitro, we studied the influence of other antimalarials (quinine and mepacrine) in mice with induced Ehrlich ascites tumour (EAT) to find out whether variations in oxygen consumption affected the course of the disease. In vitro data, obtained by a polarographic technique, indicate that primaquine and, in particular, mepacrine increase EAT-cell oxygen consumption, while in vivo data, obtained in mice injected with an inoculum of about 1 x 10(6) tumour cells per mouse, show that both drugs, but notably mepacrine, accelerate tumour growth, as monitored by Cox's statistical method for body weight, and lead to earlier death. In cases of existing neoplasia, therefore, the potentially toxic effects of certain antimalarials must be borne in mind.