Three different nitrogen forms; nitrate, ammonium and organic nitrogen with 140 kg ha(-1) dose, were compared for lettuce growing in the soil under plastic covered greenhouse during winter under Mediterranean climate. Although there was not significant yield difference among the nitrogen forms, organic nitrogen yielded 9.4% and 5.6% higher yields comparing with the ammonium and nitrate forms, respectively, and nitrate form yielded 3.6 % higher than that of ammonium. Lettuce quality properties such as weight, height, circumference, mineral content and nitrate accumulation were not significantly different in nitrate, ammonium and organic nitrogen fertigated plants. Vitamin C content was the highest in lettuce that fertigated by organic-N. Leaves nitrate accumulation levels in the different nitrogen forms were not harmful in the experimental conditions. The internal leaves of the lettuce had higher N, P, Zn and Cu than that of external leaves whereas K, Ca, Mg, Fe and Mn contents were higher in external leaves than that of internal leaves. The organic nitrogen source, (trade name Pattrone 10% N), can be adequately used as a sole N source in "lettuce organic farming" under protected cultivation, Since the yield and lettuce quality parameters were comparable to regular lettuce growing protocole using inorganic-nitrogen sources.