The objective of this study was to assess responses of certain antioxidants in harvested leaves of selected cultivars of spinach (Spinacia oleracea L.) differing in postharvest senescence rates in order to explore the significance of these antioxidants in postharvest senescence regulation and dynamics. Ten cultivars were grown in both field plots and laboratory growth chambers, harvested at maturity, and their leaves detached and stored at 10 °C in the dark. Following postharvest analysis, two cultivars were identified consistently as having relatively high (`Spokane F1') and low (`BJ 412 Sponsor') postharvest senescence rates. These two cultivars were then grown in a growth chamber for 45 days and their leaves detached and stored as above. At the point of harvest (day 0) and on days 4, 8, 12, 16, and 20, samples were analyzed for activities of ascorbate peroxidase (ASPX; EC 220.127.116.11), catalase (CAT; EC 18.104.22.168), and superoxide dismutase (SOD; EC 22.214.171.124), and (ii) concentrations of malondialdehyde (MDA, an indicator of lipid peroxidation), total ascorbate, reduced ascorbate (AsA), oxidized ascorbate (DAsA), total glutathione, reduced glutathione (GSH), and oxidized glutathione (GSSG). Although MDA accumulated in leaves of both cultivars concomitant with time after detachment, levels became significantly higher in `Spokane F1'. It is argued that declining activities of ASPX and levels of ascorbate and increasing activities of SOD manifested in accumulation of hydrogen peroxide in `Spokane F1', leading to a greater potential for lipid peroxidation in this cultivar than for `BJ 412 Sponsor'. SOD activities and glutathione levels may have increased as a result of elevated oxidative stress in `Spokane F1'. Increased hydrogen peroxide accumulation in `Spokane F1' relative to `BJ 412 Sponsor' may have contributed to an increased rate of senescence in the harvested leaves of this cultivar.