Long-term observational data are used to compare and analyze time and space variations in the concentrations of nutrients in the water of major rivers flowing into the Caspian Sea and assess the nutrients runoff into the sea. Annual variations in the normal monthly values of river runoff and nutrient compound concentrations and input into the sea are considered (18 compounds and considered for the Volga, Ural, Terek, Sulak, and Samur, and 7 compounds are considered for the Kura). The Volga contribution to nutrient input into the sea is found to vary from 77 to 94% with the average of 86%.