['Reactive oxygen species (ROS) induced lipid peroxidation is associated with sperm function. Malondialdehyde (MDA) concentration and glutathione peroxidase (GPx) activity represent the lipid peroxidation and spermicidal antioxidant, respectively. We aimed to evaluate the relationship of MDA and GPx levels with sperm parameters.', 'Specimens were divided into two groups: group 1. normospermia (n=20); group 2. oligoasthenospermia (n=31). Seminal MDA concentration was measured by thiobarbituric acid reaction method. Seminal GPx activity was measured by oxidation of reduced nicotinamide-adenine dinucleotide. Seminal MDA levels and GPx activities in both groups were compared.', 'MDA concentrations in both groups were significantly different (1.52 +/- 0.75 vs. 2.25 +/- 0.88 nM, p = 0.0021). GPx activities in both groups were non-significantly different (0.48 +/- 0.11 vs. 0.47 +/- 0.12 U/ml). MDA levels were negatively correlated with the sperm motility (MDA = -0.014 x motility + 2.62, p =0.017) and concentration (MDA = -0.0045 x concentration + 2.23, p = 0.0166). GPx activities were positively but non-significantly correlated with the sperm concentration and sperm motility.'] Is seminal malondialdehyde concentration but not glutathione peroxidase activity negatively correlated with seminal concentration and motility?