Abstract: Using orthogonal design, the effect of environmental temperature, photoperiod , splenec-tomy and oestrogen levels upon the development of exoerythrocytic forms (EEF) of P. yoelii was observed. The results indicated that a significant number of viable sporozoites were cleared in the spleen, since the EEF density was much higher in the livers of splenectomized rats (1. 73/mm3)than in sham-operated counterparts(0. 55/mm3). The effect of high level oestrogen on EEF density was also evident since there was a significant difference between experimental(0. 86) and control rats(1. 42). Low environmental temperature caused the development of EEF stunted and asynchronous,but no significant effect on the density of EEF in this group was found. The density and average diameter (AD) of EEF between rats illuminated 8 and 16 hours per day were identical. The interaction between splenectomy and elevated oestrogen level offset each other, whereas the interactions between other two factors showed no difference by analysis of variance.