Abstract: This study investigated the physiological and biochemical changes during postharvest storage of two crisphead lettuce (Lactuca sativa L.) cultivars, `Shizuka` and `Cisco`. Lettuce heads were stored for up to 10 days at 20°C. Both cultivars showed remarkable changes in weight loss and respiration rate along with hue angle value. The glutamine synthetase (GS, EC 6.3.1.2) activity decreased gradually in the leaf portion of both cultivars. However, in the midrib portion, these changes were not so prominent. Increased activities of glutamate dehydrogenase (GDH, EC 1.4.1.2) were found in both portions and cultivars with little fluctuation on 2 and 4 days. The aminating activity of GDH was substantially higher than deaminating activity. Though ammonia content was increasing throughout the storage period, significantly higher accumulation was started after 4 days. However, the initial content and accumulation of ammonia were noticeably higher in the cultivar `Cisco` for both portions. There was a highly significant negative correlation observed between GS activity and ammonia content whereas a significantly positive (highly in `Cisco`) relationship was found between GDH activity and ammonia content in the leaf portion of both cultivars. An inverse relationship between GS and GDH indicated that these two enzymes are subject to concurrent regulation by a common metabolite. Based on the parameters determined in this study, the cultivar `Shizuka` showed longer shelf-life than `Cisco`.