Nickel is a nutrient classified as a microelement, essential for plant growth, responsible for nitrogen metabolism. Research is being conducted worldwide to determine the optimal levels of nickel for the cultivation of many plant species, in various horticultural technologies. One of them is hydroponic cultivation of lettuce on rockwool. In hydroponic lettuce cultivation, the quality of the lettuce seedling is very important. Its nutritional status affects the yield of good quality and quantity. The study aimed to determine the effect of increasing nickel concentrations (increasing doses of nickel: I control, II 0.5 mg Ni dm-3, III 1 mg Ni dm-3, IV 2.5 mg Ni dm-3, and V 3.0 mg Ni dm-3) in the nutrient solution on the seedling emergence, fresh mass and chemical composition of lettuce seedlings (Lactuca sativa L) grown in rockwool. The following measurements were taken, number of seedlings emergence per treatment, total fresh mass, leaves chemical composition: N, P, K, Ca, Mg, Na, Fe, Cu, Zn, Mn, Ni and heavy metals Cd, Pb, Cr of the aboveground parts of the plant. The results of this experiment agree that application of nickel at a minute level (0.5 mg dm-3) improved the biomass of lettuce seedlings and resulted in obtaining the highest amount of N, Ca, Mg, Na, Fe in lettuce leaves. In this concentration, the disadvantageously highest content of toxic metals, cadmium and chromium was obtained. We can also agree that excess of nickel (3.0 mg dm-3) in the nutrient solution poses a severe effect on seedling emergence and fresh mass of lettuce seedlings, while also retarding the uptake of most macronutrients and micronutrients required in the plant except nickel, itself that increased when more nickel is added to the solution.