Chlorogenic acid (CGA) is a strong antioxidant that potentially reduces oxidative damage in human cells. In this study, the effects of environmental factors such as photoperiod, light quality and intensity, and CO2 concentration on the growth and CGA content of lettuce (Lactuca sativa L.) were evaluated. CGA content in fresh lettuce increased under high light intensity treatments, doubling in concentration under 200 mu mol m(-2) s(-1) compared to 100 mu mol m(-2) s(-1). Elevated CO2 concentration also increased CGA content in fresh lettuce, quadrupling in concentration when grown at 1000 ppm compared to 400 ppm. Furthermore, there was a compound effect of light intensity and CO2 concentration whereby a light intensity level of 200 mu mol m(-2) s(-1) and CO2 of 1000 ppm produced an even higher concentration of CGA, 199 mg per 100 g of fresh lettuce. Increased CGA concentration because of continuous lighting and elevated CO2 was observed under both fluorescent light and blue LED, but not under red LED treatment. Increased day length also induced higher CGA content in lettuce plants. These results show that continuous lighting, including blue spectrum and elevated CO2 concentration ca