The seasonal variations of NO₃^- discharge from forest ecosystems have been focused by many ecologists and hydrologists for their interests on its diagnostic usage for the nutrient status of the ecosystem in past three decades. Major factors controlling the seasonal patterns of stream NO3- concentration previously reported are the seasonal variations in: 1) nutrient demand of ecosystem, 2) solute transport capability of hydrological condition, and 3) in-stream nutrient usage and supply. We discussed the relative importance of these controlling factors, and tried to identify the conditions determining those relative importances, using the comparisons of the cases under different climatic and hydrological conditions, especially between the northeast United States and Japan. Seasonal variations of hydrological condition can be predominant controlling factor in the Asian Monsoon regions with summer rainy season, while the nutrient demand may be predominant in the northeast United States without significant summer rainy season. We want to emphasize that it is important to collect and organize the dataset from various climate regions for more universal understanding on the mechanisms behind the seasonal variation of stream NO₃^- concentration.