Abstract: As a cool weather crop, lettuce (Lactuca sativa L.) is very sensitive to the changes in temperature during growth. This study investigated the textural, compositional and some biochemical changes in the outer and inner leaf tissues of two crisphead lettuce cultivars ('Bittsu' and 'Cisco') harvested in different seasons. The result demonstrated that in colder months, the crispiness of lettuce leaves reduced significantly and higher amount of sugars, organic acids and ammonia were accumulated. In general, between the two cultivars, 'Bittsu' contained higher amount of sugars and organic acids, while 'Cisco' contained higher amount of ammonia. However, inner leaf tissues contained higher amount of ammonia than outer leaf tissues in both cultivars. The level of fructose was found to be higher than glucose and sucrose in all cases while malic acid was the main component in organic acid fraction. The activities of ammonia-assimilating enzymes such as glutamine synthetase (GS; EC 6.3.1.2) and asparagine synthetase (AS; EC 6.3.5.4) either decreased or nearly remain constant depending on the tissue types during the colder months. Outer leaf portion showed higher GS activity than inner leaf tissues. However, both of aminating and deaminating activity of glutamate dehydrogenase (GDH, EC 1.4.1.2) decreased in the outer leaves whereas deamination activity slightly increased in the inner leaf tissues during warmer harvest months.