Anthocyanin content and the activities of polyphenol oxidase (PPO), peroxidase (POD) and phenylalanine ammonia-lyase (PAL) and their relationships were determined in the leaves of six lettuce (Lactuca sativa L.) cultivars, exposed for 6 weeks to alternating three different day/night temperatures. Anthocyanin content was found to be highest at 20/13°C, followed by 25/20°C and 30/25°C, showing accumulation of anthocyanin at low temperatures. Activities of PPO and PAL were also found to be highest at low day/night temperatures, whereas the POD activity was decreased at low day/night temperatures. The most significant positive correlation existed between anthocyanin content and PPO activity (r² = 0.71). The results suggest that various day/night temperature regimes affect anthocyanin content and the activities of PPO, POD and PAL in lettuce.