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The extent of oxidative stress during ripening of saskatoon (Amelanchier alnifolia Nutt.) fruit was examined. Lipid peroxidation during fruit development from the mature green to the fully ripe (purple) stage was evidenced by the accumulation of ethane and 2-thiobarbituric acid reactive substances. Fruit polar lipid and free fatty acid concentrations also declined during ripening. Moreover, the double bond index of fatty acids in the polar lipid fraction fell during ripening, reflecting a progressive increase in the saturation of membrane lipids. This increase in saturation was partly due to a 65 % decline in the concentration of linolenic acid. Activities of superoxide dismutase (SOD) and catalase (CAT) fell about 4-fold and 18-fold, respectively, during development, indicating higher potential for the accumulation of cytotoxic H # O # . Peroxidase activity remained relatively low and constant from the mature green to the dark red stage of development, then increased towards the end of ripening as fruits turned purple. Lipoxygenase (LOX) activity increased 2n5-fold from the mature green to the fully ripe stage. Tissue prints showed LOX to be present throughout fruit development and Western analysis revealed that the increase in activity during ripening was due to increased synthesis of the enzyme. Collectively, these results provide evidence that ripening of this climacteric fruit is accompanied by a substantial increase in free-radical-mediated peroxidation of membrane lipids, probably as a direct consequence of a progressive decline in the enzymatic systems responsible for catabolism of active oxygen species. The role of glutathione-mediated free-radical scavenging was also examined as a potential system for coping with this increased oxidative stress. Concentrations of reduced and oxidized glutathione (GSSG) increased 2fold and GSSG increased as a percentage of total glutathione, reflecting the increase in oxidative status of fruits during ripening. Tissue prints of glutathione reductase (GRase) and transferase (GTase) showed these enzymes to be distributed throughout the pericarp at all stages of fruit development. GRase and GTase activities rose sharply during the later stages of fruit ripening, correlating well with substantial increases in the levels of both enzymes. Hence, the glutathione-mediated free-radical scavenging system was up-regulated towards the end of ripening, perhaps in response to the increasing oxidative stress resulting from the accumulation of lipid hydroperoxides from increased LOX activity, in conjunction with a decline in SOD\CAT activities.