The aim of our study was to assess protein content in grain and its yield for two types of hybrid maize cultivars at varied soil resources of nitrogen as well as the form of nitrogen fertilizer. Field trials were conducted in the years 2009-11 in the fields of the Teaching and Experimental Station in Swadzim that belong to the Department of Agronomy at the Poznań University of Life Sciences. Protein content in maize grain was directly proportional to air temperature while it was inversely proportional to total precipitation. The dose of nitrogen as well as the type of nitrogen fertilizer has a significant effect on protein content and its yield. The stay-green hybrid ES Paroli was characterized by a greater protein content in grain and yield of protein in comparison to the traditional cultivar ES Palazzo. In each of the tested nitrogen fertilizers a higher yield of protein was recorded for the stay-green hybrid in comparison to the traditional cultivar. For slow-release nitrogen fertilizers such as ammonia sulfate and urea the advantage of the stay-green cultivar in terms of the yield of protein was significantly greater than in the treatments in which slow-release nitrogen fertilizers were applied. Protein content did not determine the yield of grain when no nitrogen fertilizer was applied. In experiment I the best effect on the yield of grain was found for protein content in grain at the nitrogen dose of 150 kg N·ha^-1; variation of the independent trait, protein content, in 17.4% determined the yield of grain. In experiment II it was observed that at all the applied nitrogen fertilizers protein content statistically significantly determined the yield of grain. This dependence was best described by the regression equation for ammonium nitrate (NH₄NO₃).