How environmental factors modify the effect of time of nitrogen application on grain yield and yield components of rice (Oryza sativa L.) were studied under field experiments during the 1975 dry and wet seasons on a fertile Maahas clay soil (an isothermic clayey mixed Aquic Tropudalf). Two rice varieties, one early maturing (IR28) and one medium maturing (IR26), which are highly responsive to nitrogen fertilizer, were tested. These varieties were planted monthly from January through December. Three nitrogen levels were applied in a single or in split doses to examine the maximum nitrogen efficiency under different climatic conditions. Low solar radiation values during the reproductive growth periods of both varieties reduced the spikelet number per unit area. IR26 rice, with an excessive number of spikelets per panicle, was more sensitive to solar radiation. It produced a high percentage of unfilled spikelets at low solar radiation. Nitrogen levels higher than 120 kg N ha−1 were required to obtain maximum grain yields from both varieties under high solar radiation values during the reproductive growth periods. At low solar radiation levels, 60 kg N ha−1 was adequate to get an optimum grain yield with high fertilizer efficiency. Increased grain yields greater than 1.0 t ha−1 were obtained when the nitrogen fertilizer was applied at appropriate times. Proper application minimized the adverse effects of low solar energy.