Chloroquine resistance was induced in Plasmodium yoelii nigeriensis by the method of drug pressure. Twenty five albino mice were used at each passage and each of them was inoculated intra-peritoneally with approximately 10(5) erythrocytes infected with the parasite. The first fifteen mice divided into three groups of five mice each were treated with different sub-curative doses of chloroquine which usually cleared the infection for a few days before recrudescence. Inocula for passages to the next set of mice was made from the recrudesced infection. The sub-curative dose was progressively increased as rapidly as the build-up of parasitaemia permitted. The last ten mice divided into two groups of five mice each were used in the 'two percent test' to assess the level of resistance in the parasite. Full resistance to the maximum tolerated dose of chloroquine to the host mouse (80 mg/kg body weight) was reached at passage 20.