Abstract: In splenectomized mice, the parasitemia reached a peak level 6 days after inoculation of 3 ×107-4.5 ×107 Plasmodium yoelii parasitised cells and remained at a high level up to 20 days within which 7 out of 10 test animals died, while all the nonsplenectomized animals survived the peak level of parasitemia by day 22.Two to four days after blood inoculation intact mice were found most infective to the anopheline mosquitoes with a gland infection rate of about 40%. The mosquitoes fed on splenectomized infected mice during the same period developed a higher gland infection rate. As the time of the experiment went on, more statistically significant differences in rates between the two groups of mosquitoes became evident (P0.001).In comparison with the control given glucose only more oocysts of the parasite developed in the mosquito vectors receiving PABA of various concentrations (0.5%, 0.05%, 0.005%) in glucose solution. 0.05% PABA was found to be most suitable for the development of the oocysts.The peak level of the infection rate of the vector fed on mice with blood induced infection varied with the time of blood inoculation. It occurred at about midnight (20:00-24:00) when inoculated at 12 o'clock; conversely, it did in the morning (8:00-12:00) when inoculated at 24:00.