Different approaches can be taken to obtain an insight into the physiological basis of genetic improvement of corn. The first approach has been taken in our review of the physiological basis of genetic improvement in corn. Genetic improvement of corn hybrids in the United States has been associated with an increase of the plant density at which maximum grain is attained. Both practical (i.e., economic yield) and physiological considerations favor an analysis of genetic improvement in corn at the optimum plant density. Interpretation of the frequently complex hybrid X environment X plant density interactions among old and new corn hybrids can be facilitated by a basic understanding of the physiological basis of the plant-density response. As reported earlier, several experiments were carried out in Ontario to identify the physiological basis of genetic improvement of corn hybrids over the last three decades.