In an experiment to study time factors and quantity of Atabrine for inducing chronic Plasmodium berghei infections of mice, groups of mice inoculated with P. berghei were treated with a single injection of 0.5 mg of Atabrine, one group on each of the fourth, sixth, eighth, tenth, twelfth, fourteenth, sixteenth and eighteenth days after infection. In a second experiment, groups of similar mice were treated with 0.5 mg of Atabrine per mouse per day for 5 days, one group on each of the first, second, third, fourth, fifth, sixth, seventh, eighth and the ninth day after infection. In both experiments surviving mice were given a challenging inoculation of P. berghei to determine that they had been infected. The results confirm previous observations that mice develop chronic P. berghei after treatment with Atabrine and are immune to super-infection. It was also shown that exposure to P. berghei infection for 6 days or more produced more chronic infections than did exposure for shorter periods. Further, it was shown that a longer period of treatment also enhanced the development of chronic infections. It is suggested that the susceptibility of mice to P. berghei is attributable to the inability of this animal to withstand the burden of parasitism rather than the inability to develop acquired immunity to this parasite.