       www.ajpct.org Original Article                         1 Dept. of Physiology, Pharmacology and Biochemistry, College of Veterinary Medicine, University of Agriculture, Makurdi, Nigeria 2 Dept. of Pharmacology and Toxicology, Faculty of Veterinary Medicine, Ahmadu Bello University, Zaria, Nigeria ! The study was conducted to determine the effect of subacute chlorpyrifos exposure on some hormonal and semen characteristics and, the ameliorative effects of melatonin. Twenty four adult male Wistar rats divided into 4 groups of 6 animals each were used for the study. Rats in group I (S/oil) received soya oil (2 ml/kg) while those in group II (Mel) were given melatonin (0.5 mg/kg). Rats in group III (CPF) were exposed to CPF only (8.5 mg/kg ~ 1/10 th of the LD 50 ) while rats in group IV (Mel + CPF) were pretreated with melatonin (0.5 mg/kg) and then exposed to CPF (8.5 mg/kg), 10 minutes later. The regimens were administered by gavage once daily for a period of 28 days. At the end of the study period, the rats were sacrificed, and sera obtained from the blood samples were analyzed for testosterone, follicle stimulating hormone (FSH) and luteinizing hormone (LH). Testicular weight, testicular acetylcholinesterase (AChE) activity, epididymal sperm count and sperm motility were also evaluated. The results showed that pretreatment with melatonin ameliorated the CPF induced alterations in epididymal sperm count, sperm motility, testicular weight and levels of testosterone, FSH and LH. In conclusion, subacute CPF<induced alterations in sex hormones and semen characteristics were ameliorated by melatonin probably due to its antioxidant property. "#: Chlorpyrifos, Melatonin, Follicle stimulating hormone, Luteinizing hormone, Testosterone, Semen characteristics. $%!&’$&% Pesticides usage has increased rapidly in the last fifty years due to intensification of farming in order to obtain higher yields. The over<dependence on chemicals not only resulted in a high cost of    Dept. of Physiology, Pharmacology and Biochemistry, College of Veterinary Medicine, University of Agriculture, Makurdi, Nigeria. ()* angeez911 @yahoo.com