The female reproductive function is under the control of the hypothalamus and the pituitary. The synchronous secretion of oestradiol, progesterone, luteinizing and follicle-stimulating hormones are essential for the reproductive functions in female rats. Pesticides among other factors like drugs, environmental factors, lifestyle and diet are capable of having adverse effects on reproductive functions. Pirimiphos-methyl, a broad spectrum, non-cumulative organophosphate insecticide is widely used especially in Africa to protect food against pests. Since the female reproductive function is under the control of the hormones, this present study was designed to evaluate the effect of pirimiphos-methyl on hormone concentration during the estrous cycle in female Sprague-Dawley rats. Rats (150-190g) were divided into groups A, B, C and D of 24 rats per group and daily administered with pirimiphos-methyl (0, 10, 60 120 mg kg -1, orally) respectively for a period of 21days. Daily vaginal smears were performed to determine the pattern of the estrous cycle. After 21 days, rats were sacrificed and blood was collected at the various phases of the estrous cycle for hormonal assay. Values are means ± SEM compared by one-way ANOVA. Results show alteration in the level of hormones during estrous cycle as shown in Table 1.The result suggest that administration of pirimphos-methyl caused an imbalance in the serum hormone levels thus leading to irregularity in ovarian function and alteration in the duration of estrous cycle.