The pharmacological properties of essential oils have been widely studied. However the interaction and contribution of the individual constituents assayed singularly and in combination remains relatively unexplored. To investigate these possible interactions, various essential oil constituents (EOC¢s) were combined and the interactions on their antimalarial and toxicity properties determined using the tritiated hypoxanthine incorporation and the tetrazolium-based cellular viability assays, respectively. In combination, two inactive EOC’s (IC50 values greater than 1 mM), ρ-cymene and carvacrol interacted in a synergistic manner (∑FIC = 0.02) against a chloroquine-resistant strain of Plasmodium falciparum. This interaction was comparable to that between the potent E- and Z-(±)-nerolidol (IC50 value: 0.9 ± 0.3 µM) and the standard antimalarial, quinine (IC50 value: 0.13 ± 0.04 µM) (∑FIC = 0.01). However, this latter combination also potentiated the toxicity (∑FIC = 0.001) of each molecule. A similar increase in toxicity was noted between ρ-cymene and γ-terpinene, as well as E- and Z-(±)-nerolidol and (-)-pulegone. These results show that the pharmacological activities of individual EOC¢s can vary greatly when used in combination, and show potential as adjuvants in the elimination of malaria infections.