Abstract: Chlorination constitutes a practical and economical chemical control method for the disinfection of recycled nutrient solutions in soil?less growing systems. Although the chlorination can prevent the development of pathogenic organisms, the use of inadequate doses of chlorine could produce damages to the culture and environment. It is necessary to select doses for each plant species that do not cause damages nor produce undesirable effects on the productivity and quality. Zinnia sp. in South America has large potential for cultivation as an ornamental potted or vase flower. Tests for disinfection of the recycled nutrient solution were performed with different chlorine quantities (control, 11, 22, 44 and 88 mg L^-1) to evaluate the potential phytotoxicity and effects on the flower production (weight and number) of Zinnia elegans var. Enana. The production and phytotoxicity were analyzed in relations with the contents of macronutrients (N, P, Ca, and K), sodium and chlorides levels in leaves and related chemical changes (pH, EC and chlorides) in the nutrient solution. The results showed improvement of the development of foliage, roots and the production of flowers with the doses of 11 and 22 ppm, associated to a minor toxicity. The larger doses did not surpass the toxicity levels, although affected the productivity and quality of plants. These results enabled us to select doses under the value of 22 ppm for futures effectiveness test to control pathogens.