Pyrethroids are synthetic compounds chemically derived from the natural pyrethrins which are the insecticidal components of the pyrethrum extracts of chrysanthemum flower. The aim of this study was to investigate the toxicological effects of pyrethroid insecticide "Mortein" (M) on the bone marrow and spleen of adult Wistar rats. Twenty five adult rats (150-200g) were purchased from the animal facility of the College of Health Sciences, Delta State University, Abraka and assigned into five groups (n = 5). Groups I, II and III were treated orally with 2250mg/kg of Mortein insecticide suspended in olive oil once daily for 7, 21 and 40 days respectively. Groups IV and V were administered tap water and olive oil respectively, once daily throughout the duration of experiment. At the end of each respective durational treatment, the rats were euthanized. The spleen and bones were harvested and processed for histological analysis using standard techniques. The resultant sections were stained with Haematoxylin and Eosin (H&E) and analyzed with the aid of a light microscope for histological changes. Histological examination of the bone marrow of the treated rats revealed extensive congestion that was time dependent as well as hypo cellularity when compared to the oil and normal control groups. The rats" spleens were markedly congested and there were intra-parenchyma hemorrhages of varying degrees compared to the rats of control groups. M insecticide at the dosage considered, was highly toxic to both spleen and bone marrows of rats for the different durations considered and the effects observed were time-dependent.