Abstract: Disinfection by chlorination was applied to the solution of a soilless closed system of Lactuca sativa varieties, Gallega and Mantecosa. The aim was to study the effect of the addition of different doses of chlorine on the production (fresh weight and dry matter), quality (nitrates, vitamin C and nitrogen contents), and phototoxicity to lettuce (chlorosis) with regard to the chemical properties of the solution (pH, electrical conductivity EC and chlorides). Four treatments were applied: 0.55, 5.5 and 11 mg L^-1 (ppm) of chlorine and a control without addition of chlorine. The 11 mg L^-1 treatment produced a decrease in production of Gallega, presenting a 40 % lower fresh weight than the control. Both varieties presented high contents of nitrates. Gallega presented the maximum values (2920- 8158 mg kg^-1) and showed values under the permissible limit with the 0.55 and 5.5 mg L^-1 treatments. Mantecosa showed lower maximum values (3787- 5291 mg kg^-1), although with all the values above the limits of permission. The contents of nitrogen for both varieties exceeded the levels of sufficiency in all the treatments. This fact was related to the high nitrogen supply provided by the fertirrigation that contributed to the high nitrate contents. Gallega presented larger contents of vitamin C (19.3-28 mg.100g^-1) than Mantecosa (15.3?19.98 mg 100g^-1). Chlorination did not affect the chemical properties of the solution (pH and EC remained between the appropriate range for the species). Chloride contents in the nutrient solution were larger at the 11ppm doses; however the values remained under the toxicity levels for the species. For both the varieties, 0.55 mg L^-1 treatment produced the higest fresh weight and vitamin C contents and the lowest nitrate contents and toxicity symptoms (chlorosis), while 11 mg L^-1 treatment resulted more chlorosis and necrosis of leaves, diminishing the commercial quality of the plants.