The aim of this study was to investigate the effects of methionine and dithioerythritol, added to the Tris extender, on ram sperm motility and LPO (lipid peroxidation) and antioxidant capacities during liquid storage up to 72 h at 5 degrees C. Ejaculates collected from five Merino rams, were evaluated and pooled at 37 degrees C. This study included two experiments. In experiment 1, each pooled ejaculate was divided into four equal aliquots and diluted (37 degrees C) with the base extender, containing 0 (control), 1, 2 and 4 mM methionine, at a final concentration of approximately 4 x 10(8) sperms/ml (single step dilution), in a 15-ml plastic centrifuge tube. In experiment 2, dithioerythritol, at concentrations of 0 (control). 0.5, 1 and 2 mM, was used as an additive in the extender, and the procedure explained above was applied for the division of aliquots and the dilution of semen. Diluted semen samples were kept in glass tubes and cooled from 37 to 5 degrees C in a cold cabinet, and maintained at 5 degrees C. Sperm motility and LPO and total glutathione (GSH) and glutathione peroxidase (GPx) capacities were determined at 5 degrees C for periods of 0, 24,48 and 72 h of liquid storage.