Two field experiments were carried out on at Baramon Research Station- Horticulture Research Institute during the winter season 2009/2010 and 2010/2011 lettuce to improve the growth, head and seed yield by enhancing the metabolic processes. Two weeks after transplanting, plants were sprayed (spraying were repeated every two weeks), the treatments were as following: seaweed extract, humic acid complex, NPK (8-8-8), amino acids mixture, Ascorbic acid (VC), Vitamine E+ Selenium and control. The effect of these treatments on vegetative growth, biological constituents, photosynthetic pigments, flowering, head and seed yield were studied. The results can be summarized as following: most treatment surpassed the control in all studied characteristics, as they promoted lettuce performance and yield under the experiment conditions. The best treatment was the seaweed extract; it significantly increased most of vegetative characteristics, biological and chemical parameters which led to the increase of head and seed yield in the two seasons of the experiment. It resulted in 50% increase in head yield and 100% increase in the seed yield. Therefore, it's recommended to spry lettuce plants with seaweed extract (5ml L^-1 ) four times during the season in order to get the highest head and seed yield