Two cultivars (Karlo and Rosanna) of greenhouse lettuce were grown under different photosynthetic photon fluxes (PPF) and photoperiods provided by 400-W high–pressure sodium lamps. Natural light was compared to suppletmental lighting treatments providing either 50 or 100 μmol m-2-s-1 for photoperiods of 16, 20 or 24 h. Lettuce plants were grown in hydroponic gulleys using a standard nutrient solution. Plant fresh weights were measured every week for the duration of each culture grown between August 1989 and June 1990. The incidence of tipburn and the overall quality of the shoots were determined at the end of each crop. Leaf nitrate contents and nitrate reductase activity were measured for various lighting treatments. The highest fresh weight was obtained for the highest PPF and the longest photoperiod. However, these treatments were associated with a higher incidence of tipburn. Supplemental lighting reduced the leaf nitrate contents and affected the nitrate reductase activity.