The effect of nitrogen applications and population densities on grain and dry matter yield, leaf area and agronomic traits of three commercial corn hybrids was studied at two locations in Puerto Rico. Treatments included sidedressed N at 0, 60, 120, 180, and 240 kg/ha and three population densities of 20,000, 40,000 and 80,000 plants/ha. For most traits, significant differences were found between locations, hybrids, population densitites and N levels, and there were significant interactions of location X hybrids, location X population densities, hybrids X population densities, location X N, hybrids X N and population density X N. Grain yield increased significantly with N rates. Pest results were obtained with 120 kg N/ha and 40,000 plants/ha. Over-all grain yield was higher in Sabana Grande, a drier southern location. Highest grain yield (8,607 kg/ha) was obtained with Pioneer Brand Hybrid 304C with 40,000 plants/ha and 120 kg N/ha. Highest dry matter yield (9,393 kg/ha) was obtained in Isabela, a more humid northern location, with Pioneer Brand Hybrid 5800 at 40,000 plants/ha and 120 kg N/ha. At both locations over-all leaf area was greatest with application of 120 kg N/ha.