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Cardio-genic disruptions of in utero exposure of a non-fluoridated Cypermethrin (CP) and a fluoridated λ-Cyhalothrin (LC) type 2 pyrethroid insecticide were explored in albino mice. Pregnant dams were divided into three groups (n=10); 1) Vehicle Control Group (VCG) (intra-gastric treatment of 0.1mL corn oil); 2) Cypermethrin Group (CPG) and; 3) λ-Cyhalothrin Group (LPG) (intra-gastric treatment of 5mg/kg Cypermethrin (CP) and λ-Cyhalothrin respectively in corn oil). These treatments were applied on 7--10th days of pregnancy. Embryos were removed from the gravid uteri of the euthanized dams on 18th day of pregnancy and fixed in formyl ethanol for 24hrs to finally remove their hearts for histological and histometric analyses. Both insecticides were found to cause micro-anatomical derangements like decreased ventricular wall thickness, rudimentary appearance or complete absence of inter-ventricular (AV) septa, abnormalities of the cardiac valves, and decreased thickness of the muscular layer of the aortic trunk. However, these disruptions were much more pronounced in LCG. Morphometric results revealed a significant decrease (P>0.05) in the thickness of left ventricular wall in CPG (295.90±12.3µ) and LCG (104.27±5.6µ) as compared to VCG (346.68±9.5µ). The mean thickness of interventricular septum was significantly decreased in CPG (440.03±20.4µ) and LCG (214.3±10.2µ) groups than the VCG (2641.75±186.2µ). The thickness of the aortic intima also showed a significant (P>0.05) decrease in CPG (12.70±0.25µ) and LCG (11.03±0.3µ) than the VCG (18.44±0.37µ). Similarly, the thickness of aortic adventitia remained significantly (P>0.05) lower in CPG (26.96±1.1µ) and LCG (23.97±1.5µ) than the VCG (41.09±2.1µ). The relative abundance of myocardiocytes and cardio-myoblasts (per 5000µ2) also showed significant decrease in both CPG (8.98 ±0.45, 2.50±0.24 respectively) and LCG (4.7 ±0.48, 6.26±0.79) to the VCG (18.19±0.56, 1.02±0.13). The results show that type-2 pyrethroid insecticides inflict a highly toxic impact on the developing heart in mammals and the fluoridated LC is more toxic than the CP in this context. [ABSTRACT FROM AUTHOR]