Diazinon (Dz) is used in ectoparasiticide formulations for external parasitic control, resulting in environmental deleterious effects on biological systems. Thus we aimed to investigate the effects of different doses of diazinon on some biochemical parameters and histological alterations in female rats. The rats were divided into two groups. The first group was used as control. The second group was divided into four subgroups that were treated with 8, 10,12 and 20 mg/kg BW of diazinon, respectively. The results showed that treatment with Dz induced significant (p < 0.05) increases in the level of serum malondialdehyde (MDA) and the activity of lactate dehydrogenase (LDH). The results revealed a significant (p < 0.05) decreases in the activities of serum acetylcholinesterase (AChE), glutathione peroxidase (GPx) and superoxide dismutase (SOD). Meanwhile, the results showed significant (p < 0.05) increases in serum total lipids, total cholesterol, triglycerides, high density lipoprotein (HDL-C) and low density lipoprotein (LDL-C) in Dz-treated subgroups, compared to the control group. The histological analysis of cardiac and skeletal muscle fibers demonstrated large areas of degenerating muscle fibers with evident loss of transverse striations and wide interfascicular spaces. In conclusion, Dz induced varying degrees of oxidative damage and histological alterations according to its dose. (C) 2010 Elsevier B.V. All rights reserved.