Abstract: The effect of inhaling mosquito coil smoke on the haematology and histology of the rats spleen was studied. A total of 30 albino rats of the Wister strain were used in this study, they were divided into six groups of five rats each. Rats in Group I served as control (no exposure to mosquito coil smoke). While Groups II-VI were exposed to mosquito coil smoke for 12 h, 7, 14, 21 and 28 days, respectively. At the end of each experimental period, blood was collected from each rat for the analysis of Red Blood Cell (RBC) count, White Blood Cell (WBC) count, Haemoglobin (Hb) concentration, Packed Cell Volume (PCV) and the percentages of Neutrophils, Monocytes, Eosinophils, Basophils and Lymphocytes. The rats were then sacrificed and the spleen obtained, was processed for routine histological analysis. Haematological analysis of the blood obtained revealed a significant (p<0.01, 0.05) increase in WBC count in all exposure periods, while analysis of differential leucocyte count revealed a significant (p<0.05) increase in basophil and lymphocyte percentages. Histological analysis of the spleen tissue revealed severe congestion of venous sinusoids, hyperplasia and regression of both the red and white pulps. Results from this study demonstrates that mosquito coil smoke inhalation challenges the immune system in experimental rats, however, the precise mechanisms remain to be clarified in more detailed studies.