The influence of combinations containing the blood schizontocides chloroquine (CQ) or mefloquine (MEF), together with the 8-aminoquinolines (8AQ) primaquine (PQ) or the new, long-acting compound, tafenoquine (TAF), on the rate of selection of resistance to the individual compounds was examined using the asexual, intra-erythrocytic stages in rodent malaria models. The two main procedures used were a 'serial technique' (ST) and the '2%- relapse technique' (2%RT). The ST provided evidence for the contention that a combination with PQ slowed the selection of resistance to CQ or MEF; it has been shown previously that synergism exists between CQ and either PQ or TAF in rodent malaria. Data obtained with the 2%RT, and three parasite lines derived from Plasmodium berghei N (the 238B line), P. chabaudi ASS (the 238C line) or P. yoelii ssp. NS (the 238Y line), indicated that resistance to TAF used alone is acquired rapidly under drug pressure and that this resistance is stable when selection pressure is removed. In the 2%RT, resistance to CQ developed when another line of P. chabaudi (AS15) was exposed to that compound alone, although more slowly than the development of resistance to TAF in the 238C line. However, treatment of a TC line of P. chabaudi, developed in a 2%RT using a combination of CQ with TAF, led to little resistance to either compound. A totally unforeseen phenomenon was the appearance of a high level of resistance to CQ in the 238C line of P. chabaudi that had been exposed only to TAF; this was not observed with the 238B or 238Y lines. Attention has been refocused recently on the use of 8AQ for prophylaxis in man. It remains to be determined if resistance in the asexual intra-erythrocytic forms is carried over to the other stages of the malarial life-cycle, especially the hepatic, pre-erythrocytic schizonts. The implications of the present results for the possible clinical deployment of 8AQ in the future are discussed. It is concluded that, whereas use of an 8AQ alone carries a high risk of selecting resistance, combinations containing 8AQ may have a place in the protection of blood schizontocides that are to be deployed in endemic areas. Furthermore, the inherent gametocytocidal action of the 8AQ should promote the reduction of transmission.